DISCLAIMER: The whole question has spoilers about Lost Mine of Phandelver.
So, I went through the Redbrand Hideout with my party. They
entered by the tunnel (so, straight to area 8), fought and defeated the Nothic.
This fight was the one they used most resources – two 1st level spell slots from the 2nd level Cleric for healing, Lay on Hands from the Paladin for healing (7 HP out of the 10 total HP pool), Hunter's Mark from the Ranger for damage. Also, the Paladin used one divine smite.
After that, they
found the secret door to area 12 through area 7, surprising Glasstaff. Even though he was surprised, I had pre-cast Mage Armor on him, making him AC 15, and gave him the reaction (yeah yeah I know surprised doesn't have reaction) for using the Staff of Defense for Shield (+5 AC).
Our Druid used Faerie Fire on the enemy, as the first person moving. He failed the ST, making every other attack against him have advantage. Our Paladin had the +1 Longsword from
the Nothic hoard treasure (they jumped the hole).
He critted, with divine smite, dealing 6d8 + 4 (31 average) damage. So… yeah, Glasstaff was unconscious before being able to make any action or reaction.
Is the fight (and the dungeon as a whole) supposed to be that easy? I understand that they skipped essentially every content on the dungeon, making literally the fastest path to Glasstaff they could make, but still… Everyone's left with a feeling of "really? Was it supposed to be that easy? I'm confused."
I'm running this adventure for the first time, so I'm not sure about: did I do something wrong? – did I misplay Glasstaff or something? Were they just too lucky in finding the secret door, getting to surprise him and critting him?
Am I mistaken and the real challenge begins now, as they are mostly drained out of resources, and there's still much dungeon to explore?
As a note: Glasstaff roled for an amazing 3 initiative, so even if he wasn't surprised, he would be unconscious (PCs didn't kill him) before his first round anyway.
I am sincerely confused about how easy this "dungeon" and its "boss" was. As I said, the main "questions" (I think they are all related, so no "more than one question" here) are:
- Is this supposed to be this easy?
- If it is not, did I do something wrong?
- If it is, is there something I can change (the next time) so it actually becomes more challenging?
I'm aware that Glasstaff encounter gives only 200 XP, being a CR1 monster against 4 2nd level characters, so pretty easy encounter. But should it have been different, overall?
And, as I mention in the body
- Am I mistaken and the real challenge begins now, as they are mostly drained out of resources, and there's still much dungeon to explore?
Probably implied for now, but the party this time had only 2nd level characters, one Druid, one Paladin, one Ranger and one Cleric. To be fair, they aren't even worried about optimization.
I don't feel like I have played the NPCs/monsters wrong. As I said, the Nothic itself drained a lot of resources from the party – 2 out of 3 spell slots from the Cleric, 1 out of 2 spell slots from the Ranger, 1 out of 2 spell slots from the Paladin, dropping the Cleric and the Paladin to half HP and the Ranger to half HP as well.
Glasstaff, on the other hand, with his miserable 22 HP and (even with) 20 AC, got pretty easily hit by the +6 attack bonus from the paladin (+1 LS) (it actually critted, as I said, so that doesn't even matter) and +7 attack bonus from the Ranger (+2 from archery, +5 from usual).
No, you did nothing wrong.
There are a few things here that stand out to me regarding the Glassstaff fight on which I can elaborate a bit.
Surprise is very powerful
Average damage and healing numbers compared to average hit point pool sizes just mean that the most effective way to win a fight is simply to put out more damage faster than the opposite party. Surprise helps a lot with this.
Smite + Crit = Ouch
Paladins are notorious for their crit smites. It is indeed correct that a crit will also double the damage dice of a smite, so when it happens there's not much you can do but smile, be happy that your paladin got his moment in the spotlight, and say goodbye to your monster. This is a big part of why some people play paladins and it does use quite a bit of their daily resources.
Low level encounters are very swingy
Low level encounters are in my experience quite hard to balance, simply because your players' resources are still very limited; Their hit point pools are very small, so a single (un)lucky roll from a monster can knock them out. They have few spell slots to mitigate incoming dangers. They have only one attack per turn and low attack bonuses, so a little bit of bad luck means the whole party might put out no damage at all for a whole round. This means that you have to be very careful with setting your monsters hit point maximum and average damage output, or a fight can turn very sour very quickly - at low levels at least. This also means that it's generally better (depending on your group's desired playstyle of course) to make encounters a bit easier than normal.
The action economy
5e takes great care to make sure that the action economy is kept stable, that means creatures (monsters, npcs and players alike) must have a very good reason to be able to make more than one action per turn (with the exception of legendary and lair actions for higher level boss monsters, which only exist to fix exactly the problem in this case). This, again in combination with the way the combat works (more damage faster wins), means that a group of four versus a single enemy has a huge advantage, because the single enemy has only a quarter of the actions available and is a lot more vulnerable to bad luck.
What you could do differently next time
Make it harder to surprise the enemy
Maybe the wizard had cast alarm on the other side of the secret door, so he knows someone is there, or make the secret door harder to find.
Dynamically scale up a monsters max hit points
This is something I do a lot. My players usually play pretty optimized, so if it looks like an encounter is about to face an abrupt and anticlimactic ending, I simply retconn a monsters hit point maximum to a higher value that is still within the hit dice range. So for example "Glassstaff", who uses the statblock of an evil mage has 5d8 hit points, or 22 on average. In this case I might have probably just increased his maximum to the actual maximum, 40 (5 * 8), which would have left him with 9 hit points after an average critical smite, so he can at least act once.
Balance the number of opponents
After all, that's what the guard quarters two doors further down is meant for. Either have a few guards in the room already to deliver a report or whatever to their boss, or have them join the combat midfight, because they might have heard a noise. Although, if your players made sure to prevent that scenario, either by clearing out nearby guard quarters or eg. casting silence to muffle the combat noise, make sure to give them the advantage of such careful preparation.