Easy method
Time to do 26 minutes.
Level Il molise bonus level.
Settings:endless arcanas and hurry on.
I KEPT DYING ON EVERY OTHER LEVEL WASTING 25 MINS EACH TIME.
This one the enemies don't move as they are plants, actively farm the gems and keep levelling up any of the weapons you find, doesn't matter too much what you pick.
When levelled enough to can just run in one direction until you hit the 100k mark then start, options quit and boom achievement.
I watched netflix while rubber banding in one direction.
So easy as the start weapon is weak on every other level against moving enemies.