Respawn enemies?

[Breach]

No, they don't respawn automatically, but:

1. Some new enemies are quest / dialogue related. E.g. in the case with the Dragonflies -- if you kill them all, then go to the woman in the village he would exchange wings for crystals and ask her where to find dragonflies, two more stacks will again appear near the lake. Also in the Dwarven lands, there's an escort mission. Even if you clean up the path in advance, several new enemies will appear when you accept the mission.

2. Some armies go stronger, the stronger you grow (I think). E.g. the cursed kid's army keeps getting stronger, the higher level you are (I think).

[maltz]

Yep. After finishing the game twice I didn't find any re-spawn other than the quest/event related ones.

The enemy's size is fixed - they never grow. I guess this makes sure that everybody can eventually win the game.

Another interesting fact - no matter how early I tried to take Learning Lv3, I always have barely enough EXP to reach level 30 just before the end on normal and hard. So maximum lv 31 is a lie.

[Metathron]

Yeah, level 31 is impossible, it requires 1,000,000 experience, and up to level 30 you've accumulated 500,000-something experience.