ToME: the Tales of Maj'Eyal

Are they earned by continuous time spent on the level?

Or are you allowed to leave and come back and keep earning them?

Also, if you leave and come back, do the enemies stay as strong as you built them up to? Or do they return to the base level they should normally be at when you start it?

Meaning if I hang around until monsters spawning there are 20 levels higher that they started and leave, will they still spawn at 20+ levels when I come back? Or will they spawn at the base level again and I'd have to build them up again?

The power of the slimes remains saved with the rest of the level when you leave, along with your progress towards the achievement.

EDIT: Though the 20+ levels seems less important than the fact that, by that point, I believe something like 30% of the slimes are randbosses, and the rest are elites.