Damagedie:
Very Weak – d4
Average/Weak – d6
Average/Strong (default) – d8
Very Strong – d10
Ridiculous (Maximum) – d12
For NPCs between these categories, use the odd numbers (d7, d9, etc). NPCs are allowed to be strong offensively (with a high damage die) if they’re weak defensively in exchange. Something strong offensively AND defensively should be some sort of special NPC – a boss on a level, a mid-boss, a quest NPC, a guardian for something, etc.
Most combat NPCs use d8. NPCs that aren’t intended to be fought (peasants) use d6.