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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.
