Extremely self explanatory- play the Mad Forest with the Inverse mode enabled. Endless & Arcanas are also helpful to activate too.
Hitting level 80 should be a breeze using a character you’re comfortable with. You can quit after you get to level 80 and the achievement will unlock. I did this with Queen Sigma and had plenty of time remaining.
Edit 1: A handful of you have recommended endless mode - definitely helps if you're not super confident with the time limit.