You cannot ensure it.
Note that the 3.5 monk was extremely weak and Pathfinder did not substantially fix that. The myriad deficiencies of the class are all working against you here.
First, every number you have, is going to be low. You need every ability score except Charisma, which means you are going to have to spread your scores very thinly. In reality, as much as you probably want Intelligence, you probably have to drop that, too. You need the Strength, Dexterity, Consitution, and Wisdom too badly. Even if you roll for stats and roll preposterously well, as you level you will be unable to afford items to increase every ability score as much as a more focused class can increase their one or two most important scores.
On top of that divided attention, you also start with low numbers: ¾ BAB, d8 HD, and no armor mean your attack, HP, and AC are going to be very low (your saves will be good, though). This is going to badly affect just about everything you do.
Second, the class features that you get simply don’t do very much for you. Having full BAB with Flurry of Blows and combat maneuvers is a nice improvement for the monk, but it’s not nearly enough. Flurry of Blows is similar to Two-Weapon Fighting, but unlike a rogue who takes that feat, your bonus damage dice grow very slowly (1d6 per 5 levels rather than every 2), and are extremely limited (monk level attacks/day; you could easily go through that in a single fight). Evasion is nice, sure, but most of the ki options are very minor, as are the numerous niche bonuses to random things (Still Mind, Purity of Body, etc.). And as nice as Abundant Step is, and Pathfinder does let you use it slightly more than 3.5, it comes late and isn’t enough.
Third, out of combat, you only get 4+Int skills, and Intelligence is your least-important stat after Charisma. That right there should tell you a lot about how out-of-combat goes: the two most important non-combat stats, Intelligence and Charisma, are the scores you have to dump. You do have a decent class skill list, but it will be hard to use: you might very well only get three or four of them. Acrobatics, Sense Motive, and Stealth are probably your best bets. None of your class features come into play substantially.
What you can do:
Find a focus. Choosing the suli race and monk of the four winds implies damage dealing, but unfortunately a monk is never going to be a major damage dealer. Keep the standard suli racial traits; being able to switch element at will is worth far more than the minor benefits of the Energy Strike option. That at least gives your damage reliability against a variety of foes. If you dump Intelligence as I fear you must, you might consider dumping Strength, as well; you’ll be hurting for feats, but if you get Weapon Finesse, and your damage comes primarily from your unarmed strike damage plus your energy damage, that allows you much higher Dexterity, Constitution, and Wisdom, helping your AC, HP, and attack. Your saves should also be quite high.
Unfortunately, you’ll be a mediocre damage dealer with little-to-no skills and no combat presence aside from that damage. Strength and Intelligence are necessary for most combat maneuvers, which aren’t great but are at least a way to have more effect on the flow of combat than just throwing some damage around. They allow you to position the battlefield a little more favorably by locking enemies down (trip, grapple) or moving them around (bull rush, reposition).
If you focus on skills, you’ll need more Intelligence, probably 14. Having 6 skill points per level is respectable for a warrior (sadly) but with that much Intelligence you’ll have a hard time being much of a warrior. You can get Combat Expertise, and Agile Manuevers allows you to use Dexterity for CMB instead of Strength. That might make you capable of doing some tripping while dumping Strength. Unfortunately, without Reach (which you can’t get if you want to use your elemental damage features), you’re going to make a very poor tripper. Combat Reflexes and Improved trip are necessary; Greater Trip is nice, since without it you can’t use your elemental damage again. If you’re only attacking Prone targets, you may be able to skate by without Weapon Finesse, at least at low levels.
The last option is to try to dump Dexterity instead of Strength. This has some advantages (fewer feats needed to make attacks or trips hit), but you lose out badly on AC and skill bonuses, since Dex-based skills are massively better than Str-based ones. Also works better with size increases, which you very much want but have no native access to. If you have allies who can offer some consistent and reliable way to increase your size, though, that could be pretty useful. It would improve both your damage and tripping if you went that way. Unfortunately, low-Dex tripping works very poorly in Pathfinder, because you need so many attacks of opportunity for it.
Ultimately, even if you pick a focus and make some tough sacrifices, you are still nothing but mediocre at what you do. And unfortunately, there just isn’t much to be done about that.
About your rolls
Your stats are preposterously good; that helps. Those are stats that can make a monk shine in early levels, and keep pace in low-mid levels. Even if other party members have similar stats, the difference between a barbarian who has Str 20, Con 18, Dex 10, Int 10, Wis 10, Cha 10 and one who has Str 20, Con 18, Dex 18, Int 14, Wis 16, Cha 12 isn’t that significant; the barbarian certainly likes the other stats but his focus is on Strength and Constitution and he can live with just those. A monk, on the other hand, really needs all of that.
The really nice thing is that it means you don’t have to make the sacrifices I mentioned, nor do you have to set feats on fire with Weapon Finesse or Agile Maneuvers. Combat Reflexes, Combat Expertise, Improved Trip, Greater Trip will allow you to lockdown the battlefield; those are probably pretty good ideas, and you can actually do all of them.
So that is definitely good. Unfortunately, by low-mid levels (particularly by 7th), spells are going to be putting all non-spellcasting classes to shame, and by mid levels (say, 11th, at the latest), the better BAB, focus, and class features of the barbarian are also going to make you look pretty mediocre in comparison.
Just so you know where you’re going. If you don’t expect the game to reach those levels, it won’t matter, and with those rolls (and a low-mid optimization group), you’ll probably be fine enough.
About your houserules
The 18-20, ×3 crit is nice (and generally unprecedented), but focusing on critical hits is far too expensive for something that just doesn’t happen enough to justify those costs. Particularly for you, who is going to need a lot of feats. Plus crits don’t multiply bonus damage dice, like your energy strike feature, so that’s pointless.
The issue with the feats is why the Weapon Specialization is not a feat you’re likely to ever take. Actually, that’s why it’s a feat no one should ever take: even a fighter has far more options that accomplish much more than just +2 damage, even with his larger number of feats. The entire Weapon Focus line is a waste of feats; you can do better. They are small bonuses to things you already do, when you need feats to give you new capabilities.
Just because it has to be said: your character does not need to take the “Monk” class to have the backstory and character you want. A cleric can take Improved Unarmed Strike, and cast spells to put elemental damage on his attacks. A psychic warrior might be even better at it. Both are mystical, Wisdom-based front-line combatants, and their other options are far superior for maintaining usefulness in a variety of situations, particularly the cleric spells that you can change every day.
Another option is to houserule judiciously. For example, what if the monk could use Wisdom instead of Strength on attack rolls with monk weapons (including unarmed strikes) and for CMB? That would eliminate a lot of the pressure to pump all your ability scores. And the monk could easily have 6+Int skill points per level, like the ranger. Full-BAB in general is another good idea; after all, combat presence depends a lot on attacks of opportunity, and the current rules deny you full-BAB in that case.
These won’t fix the monk, but they’ll help. Fixing the monk requires writing some really solid, serious class features that are unique, meaningful, versatile, and usable. The current monk class features tend to be none of these things; they tend to be harshly restricted versions of spells, and many of them aren’t even copying particularly good spells. Fixing the monk means writing a new class and calling it monk. I’d personally recommend saving yourself a lot of trouble, and using cleric, psychic warrior, or if you really want a class distinct from those, porting the swordsage from the 3.5 book Tome of Battle into Pathfinder (numerous people have already published fan-conversions of the material from that book; my favorites use Sense Motive instead of Concentration for Diamond Mind).
I wouldn't try to make his character more powerful, but give him alternative ways of helping.
There are some enemies that are inherently harder for some classes or characters to fight. That doesn't mean that they're weak, or can't assist during a fight; it just means they shouldn't be on the front lines(usually.)
A good example is creatures with magic resistance, which makes them very potent against spell casters. That doesn't mean you should make the spell casters stronger, or "more helpful," as it will end up making them even more powerful when this weakness isn't highlighted.
Perhaps the monk could body-block for one of his party members and take the Dodge action, preventing the Balor from advancing toward his allies. Effectively, he should be playing defensively, as the majority of damage is triggered by attacking the balor. This is more-or-less the intended line of play. It should be a challenge for a player to overcome.
Personally, if my player can sell me on something, I'll allow it. I actually had a fighter that doused himself with water before running into melee against a balor. I loved the idea, so I gave him resistance to fire damage for that turn, and he waylaid the balor and then fell back on his next turn.
I think good ideas should be rewarded, as it encourages outside-the-box thinking and leads to more fun moments in combat, and usually, makes the player feel like they were contributing to the encounter. When fighting deadly enemies such as this, some strategy is expected and even require.
A monk charging headlong into combat against a balor and continually soaking up damage is less of a strategy, and more of an action. Rather than simply give the character an item to resist the damage or otherwise counter the "rebound damage", give them alternative ways of contributing to the encounter.
But, that's just my 2 cents.