Not to be that guy, but the best advice really is to not play 5e. It’s a bad game, there was literally never any thought to balancing it despite being a combat focused game. Things that are meant for your character level will be unbelievably underpowered, things that are slightly too high will be a slog.
Add stuff to the scene that’s not just damage. Stuff that splits their attention and requires some strategic thinking.
- Mooks are doing a ritual that will have bad results if it’s completed in two rounds.
- Mooks flee to sound the alarm/get reinforcements
- innocent hostages are at risk
- some of them are fake hostages and will attack if “rescued”
- some of them are actually innocent, but are dominated/acting against their will
- any number of gimmicks that force people into smaller groups.
- the boss is tangible only to one player at a time. Like that time in bg2 when I cast time stop and everything stopped except my wizard, and the demogorgon.
- the boss is split into X parts that need to be killed at the same time , in separate rooms.
Also, playing Mage and Fate most recently it’s been really refreshing not having any of these DND problems.
This, this, this.
Also I often find inspiration in mechanics from MMOs, RPGs, and boss fight games like Elden Ring.
- Shifting damage resistances based on HP phases or whoever crit it last
- Phases where the boss takes no damage and the players have to do individual actions around the room before N turns or the boss heals
- Forced side battles where one or more players need to kill a mook in another room before returning to battle
- Shields that go down when players step on specific pressure plates, ensuring they’re in range of my dmg
- Capture the Flag style McGuffin delivery to kill the boss
- Boss is fleeing through a dungeon at ~1 room per 3 turns and the party has to complete room objectives to reopen doors/keep up with him/keep LoS.
- Boss has resistances to all damage until hit with N conditions when those resistances drop for N turns
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Use the maximum HP possible from the dice instead of the average given (eg. 6d12 = 72 instead of 39), or at least a higher portion of the maximum quantity
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Increase AC
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Give it extra damage of a different type
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Give non-lair monsters lair actions, and give monsters with lair actions an even stronger lair action they can use when below half-health. Same with legendary actions
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Look at older DnD editions and see if the monster or any similar monsters have extra abilities you can add
Edit: I should have specified that these are in ascending levels of difficulty for the DM, but are also more interesting
The risk in 1 & 2 is that monsters become a slog and characters are no longer heroes. If you need to hit a monster 30 times before it dies (because half misses due to AC, and you need a lot of hits due to HP), it’s just slow and boring.
Lair actions and distractions/barricades to get to BBEG are where its at. There’s a trap in the way. New support keeps popping in until something is destroyed. Something changes drastically halfway through the fight.
