Level 12 Analysis

Your AC is very low. At level 12, AC 18 is essentially a death sentence. I'm going to assume you have a source of Mage Armour from a party spellcaster, giving you an AC of 22, and a further Ring of Protection or Amulet of Natural Armour or Barkskin or something to up that by +2 to 24. 24 AC is still incredibly low.

The lowest attack bonus I found with a quick peruse of CR 12 monsters is +18, the secondary Bite of an Athach. That hits you on a roll of 6. The most common number I found was +23, which would hit you on a 2. I saw lots of +26, +28 and even a +33. Those all hit you on everything but a natural 1.

In other words, your AC is too low if anything targets you for you to survive. The DM will have to pull punches not to kill you.

Valiant Stand improves this. By not by enough. Enemies are still going to autohit you. Best case is +4 stat boosters on dex and wis, mage armour, nat armour/deflection bonuses, and valiant stand - 18 base, +4 from enhancement to stats, +4 armour, +2-+3 nat armour/def, +3 valiant stand = 32. That's a hit on a 9 from the lowest common attack bonus I found on CR 12 monsters.

Your Attack Bonus Is Pretty Decent, For a Monk. You have +16 to hit without Smite. With Amulet of Mighty Fists +2, that's +18ish to hit. With Belt of giant Strength +4 or Bull's Strength, that's +20 to hit. Everything that has AC that is supposed to be a defense has 26+, which is good - you hit most of the time. Power Attack and Flurry are counterbalanced by Valiant Stand, which you should be endeavouring to have always active by use of Stealth, which you don't have maxed out, uh, why? Popping up out of nowhere for a flatfooted smiting power attacking flurry should be your opening round action in every combat. Even your iteratives have a decent chance to hit, with this hit bonus. Only decent, though.

Your Damage Isn't Great. 1d10+1d4+5 (where is that errant +1 damage coming from?) averages out to 13 points of damage per strike. With Power Attack, that's 19 damage. With Smite, against evil or chaotic outsiders/dragons, you get /+18/, so 34 damage per strike, which is actually level 12-worthy. Too bad you get 3 total attacks per day with that damage. Sweeping Smite carries the Cha-to-hit bonus, but not the damage from the 'first hit'. It's kind of like a really terrible whirlwind attack that way.

Most of the time you'll be attacking for 19, on average. Some stuff is immune to bleed, or has DR/cold iron and good, etc, so it might be less. If you move, you only have one attack, so your first round barring stealth will nearly always be one attack at best. Rounds after that, you can flurry for four attacks, each which might hit for 19 damage.

So a max damage on an ideal round of around 34+19+19+19 damage, or 91 damage in 4 hits. That's not terrible. It relies on getting a full round action, though. It also relies on hitting with iteratives, DR of the kind you can get past, no miss chances etc. Most monsters at this level have at least 150hp, so you won't be winning in hp races. But you will be able to contribute a bit at least to the usual team of four on one beatdowns.

Your HP gets eaten in a round. With your AC, most CR9+ melee monsters are going to kill you to dead in a round. 11 Constitution on a melee combatant isn't a great idea, but then, you're a monk and need wis and and dex and str and also cha for being a paladin, so eh.

Your Saves are decent. They could be better, but they could also be worse. Most DCs seem to hover around DC 23 at this level, going up as high as 28 but also as low as 21. With your +11/+11/+13, you should have a 50% shot at most saving throws, and although fear and poison aren't exactly the most common life-threatening things to save against, it's still nice.

Power Level: Fighter.

This character can be relied upon to be roughly as good as an unoptimized fighter. The DM will have to pull some punches, and tough encounters (CR +2 or greater) will have a good chance of one-shotting this character by accident. In a party with Rogue or better power levels, this character will feel notably weaker than the rest of the group.

Note

Evaluating something without gear is hard, because at the lower end of the scale, gear can have more effect on your ultimate spread of numbers than the classes chosen. If your gear is random stuff, you will be weaker. If you plug it all into boosters for your FOUR necessary stats (dex, wis, str, cha), rings of deflection and other stuff that helps boost your melee stats to make you 'great' at that rather than 'mediocre to good', which includes buffs from friendly spellcasters, you will be stronger.

Note

This build is stronger at lower levels, where a handy Mage Armour plus your defensive stats can put you out of reach of opponents attack bonuses, and things are easier to hit and easier to kill. Level 12 is definitely where things are starting to go 'downhill', and they will likely get worse as enemy AC and AB increase and every enemy has flying or spells and abilities.

Note

The build has no movement capability other than skills-based climbing, swimming, jumping. I'm assuming again allied spellcasters will be patching that hole with buffs, especially since with the other gear i've assumed in your build you don't have the money to buy gear to get flight or teleport or whatnot.

Suggestions

Your main problem is that you're playing a monk. Without serious optimization, monks are very weak. You've done a bit of optimization here, and pushed it up to unoptimized fighter level, but fighter is still not great if you're planning to be in a party with casters. Or if you're fighting caster-style enemies like many demons are.

Most of the tricks I know to make melee characters more competitive are in 3.5e. I'm assuming there's no backward compatibility in your group. So that's all out the window.

Paladin isn't giving you much other than flavour, either. You've got a decent Flurry and Attack Bonus, but you're way too fragile to make good use of it, unless your enemies come in the 'always attack the guy using the full defense action with the tower shield' variety.

In this case, i'd recommend Summoner. Synthesist Summoner. It slices, it dices, it buffs, it has level appropriate abilities, natural attacks, the works. It is, in fact, so good that you could mix it with a Paladin dip and still be relatively awesome.

Evangelist lets you keep advancing as a Summoner while also adding a divine feel to the class. It's also the only prestige I can find that advances your eidolon aka living armour, so.

Other. than. that... there's a good rogue-based fear build, but I don't know how relevant that is to your concept.

I feel like there's some way, between Fighter archetypes and the Hungry Ghost Monk monk archetype, to create kama-wielder who reaps ki from his opponents and uses it to chain-stun them with a huge Wisdom and Dex, just whittles them away with flurries of weak, ki-reaping attacks and then spends the ki on stuns. There's too many damned archetypes though.

Even if you're taking summoner, it's kind of worth dipping monk as well. You have to be unarmoured to use your synthesism, so you could benefit from a high wisdom score to your AC. Not worth going Hungry Ghost, because the ki-drain only comes at level 5. Just a one, or at max 2-level dip. Probably after you have 1 or 3 levels of synth summoner.

Yeah I cannot find anything that is about punching people that isn't just a Fighter or Barbarian with a 2-hander sword that is any good. There's some rogue stuff that is nice, there's an alchemist archetype that gets sneak attack and would make a murderously effective hand to hand combatant with the mutagen and the buffs. But nothing based around jumping on things and punching people except synth summoner. Yeah. That's my advice.

Synthesist Summoner 20.

Optional; Monk 1.

Optional; Evangelist.

You could use a breastplate, and should use a heavy shield, but anything more than that costs you too much: you really aren’t that squishy, and you can expect things to get better on their own in a few levels anyway. Sinking more resources into it isn’t worthwhile.

All 2nd-level characters are pretty squishy.

Your AC is fairly low,¹ but 16 HP isn’t at all bad for a 2nd-level character.² For comparison, the absolute best you’re likely to see at this level is a Barbarian 2 with 27 HP.³ He had to put a lot of investment in to get that, though, and has to be in the thick of things, and does not have access to any kind of healing. You have built-in healing and (most likely) great saves, and your goal is to be hanging back anyway.

So really, you aren’t that squishy. Definitely not so squishy as to justify something drastic like dipping fighter or setting a feat on fire. You are squishy in the sense that all 2nd-level characters are, but in a level or so that will be largely mitigated.

Avoid expensive options for improving AC or HP

Taking levels in other classes or using feats are very, very expensive ways to do anything. Because your squishiness is very much a temporary problem, you want to avoid sinking resources that are particularly precious into this.

Spellcasters multiclass poorly

As a spellcaster, you do not want to fall behind on your spellcasting. Spells are the most powerful force in the game, you want to get higher-level spells ASAP. Fighter, on the other hand, is a very poor class, and is not remotely worth waiting another level for the next level of spells.

Armor Proficiency isn’t worth a feat

Full-plate armor is quite expensive at your level, has significant drawbacks in terms of armor check penalty and weight, and provides “just” +5 additional armor AC over a chain shirt. Now, +5 is a fair bit, but armor AC is of only marginal value: a lot of things just outright ignore it.

Meanwhile, feats are one of the scarcest and most valuable resources in the game. You get very few of them, and some of the things you can do with them are very powerful. That +5 armor AC is probably worth the various drawbacks that full-plate has... but only probably, not even close to being so worth it that it’s also worth burning a feat on it.

You can definitely work with what you have

Mundane items are fairly cheap, and this is what they are for. That’s where you improve your squishiness.

The primary options are, unsurprisingly, armor and a shield.

You can wear up to medium armor: medium armor, though, does kind of suck

Personally, I really dislike the medium armors. A breastplate would give you +2 armor AC over a chain shirt, but costs more, weighs a lot more, has significantly higher armor check penalty, and reduces your movement speed. If +5 is only kind of OK, you can imagine how I feel about +2. Even though you don’t need a feat for it, I wouldn’t actually get a breastplate. A (masterwork) chain shirt remains my recommendation.

If you really care absolutely not at all about any skills that take an armor check penalty, though, a breastplate can work. It does slow you down though, and you really don’t want that.

Playing with a crossbow does hurt you though: you want a shield

A crossbow requires two hands to use properly. That makes it difficult to use a shield, and shields are very good things for buffing clerics. Yes, without a crossbow you’re probably stuck doing nothing while the allies you’ve buffed finish off the opposition, but that’s a temporary situation; in a few levels, you’ll have a lot more spell slots to play with.

As you yourself say, you’re doing very little damage with your crossbow. Worse, that’s probably not going to improve much as you level up: if you think you’re doing very little damage at 2nd level, at 8th level that same damage is going to be truly pointless. Giving up a shield for this is not a good idea. If you had good Strength and Constitution, clerics can make excellent melee characters, and those clerics should use a two-handed weapon of some sort, but if you’re doing support/buffing, a shield is very-nearly-free AC, plus it’s a second platform to put magic properties on.

So ditch the crossbow, and use a shield. If you want to maintain your ability to take pot-shots, you can use javelins or some other one-handed thrown weapon. It’s not really necessary, but it can’t really hurt, either.

As for which shield, there is almost no reason for you to not use a heavy shield, but if for some reason you want that hand available, a buckler is nearly as good. You probably will never use a shield bash anyway, so it’s really just a question of an extra AC vs. having a hand free.

Positioning and tactics are crucial for everyone at these levels.

You are supporting and buffing; that means stay out of combat, keep tougher characters between you and ranged threats, and take cover when you can.

A mount can help, temporarily

Buying a warhorse is an option; they’re not very expensive, and they do provide a lot of mobility. They are also unaffected by your armor, which means you can much more easily use a breastplate if you like.

On the other hand, you have to make Concentration checks to cast spells if your mount moves the same turn you cast a spell, and that’s rough (DC 10 + spell level, when you only add +4 to the check, is a problem). You also don’t have Ride in-class, which means you only have a +2 on the check: enough to be pretty confident in your ability to move from place to place, but not to do anything fancy (even guiding it without using your hands requires a DC 5 check, which you have a 10% chance of failing).

Additionally, as a Medium human, you have to ride a Large mount – and there may easily be dungeons that won’t accomodate one, which will be very annoying.

The real problem with mounts, though, is that most of the typical ones are pretty low level. Fine for now, but don’t expect a warhorse to make it to 11th level. Even if you took Mounted Combat (which greatly improves a mount’s AC once per round), sooner or later someone will dedicate the time to get past that – and Mounted Combat, as a feat, is pretty expensive. Either way, by mid levels most attacks are pretty likely to kill a typical mount in one hit. Using a mount past the early levels tends to require that you get one as a class feature (e.g. animal companion, paladin special mount, etc.)

To add insult to injury, the recent FAQ on mounts makes them almost unusable when they’re not a class feature. No running or charging, for example. As a support/buff cleric, you may not have problems with this, but you very well might.

In the long run, your spells have got you covered

Clerics are one of the top-5 most powerful classes in Pathfinder, almost-purely on the basis of their spellcasting. Your class’s survivability is on the very high end. At very-low levels, spells are a little more difficult to use. In a very few levels, however, it will become easy to buff yourself very well.

Thus, anything you do to improve your survivability now, is probably a bad idea if you could have used that same thing to improve your spellcasting. You don’t want to multiclass, because that’s a level you could have used to improve spellcasting. You don’t want to use a feat for armor, since there are plenty of feats that make your spellcasting better.

You do have a much bigger problem

Far more significant than any “squishiness” you have noticed, is the fact that your Wisdom is quite low for a cleric. Wisdom 16 is a typical low-end value for a cleric. Wisdom 14 is very limiting, drastically reducing your access to bonus spell slots and making the saving throws against your spells extremely easy to beat.

Meanwhile, the Charisma 18 just doesn’t offer that much to you: healing in Pathfinder is weak, and that includes Channel Energy. Until you get the heal spell, healing is pretty much out-of-combat-only, barring emergencies. And out of combat, a cheap wand of cure light wounds will allow you to top off your party without touching your spell slots, and is standard adventuring gear (your party should be willing to pool funds for one; by the time you finish it, 750 gp for another should be chump change).

So generally, a cleric would prefer to have your Charisma and Wisdom swapped (or Constitution 14, Wisdom 18, and Charisma 12, honestly). Someone with Charisma 18 is usually better off in another class, that will put that Charisma to better use (read: base spells on it). It’s well beyond the scope of this question, but it may be a question you want to ask separately.

1. ^{A typical 2nd-level cleric would wear a chain shirt or breastplate, and use a heavy shield, for 16-19 AC depending on Dex and armor choice.}

2. ^{Your Constitution, on the other hand, is a little low. For preference, I’d put Constitution as second-most-important stat for a support/buff cleric, which in practice probably means 14. Losing 1 HP per level does hurt you, but it’s not critical.}

3. ^{Base 18 Constitution, +2 Con from race, maximized d12 at 1st, high-average 7 roll from 2nd. Would go up to 31 HP during the 2 rounds of Rage each day.}