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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.
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
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.
