EXTRA: Bat Country Achievement in Vampire Survivors

Mad Forest is the first level of the game, and refer to Greatest Jubilee (50G) in the base game for unlocking Inverse mode.

As the description says, you need to reach level 80, so turn on Endless to be safe. If you've already completed the base game by this point (and have Sigma unlocked), this should be pretty basic by now. Start a run with Sigma (or your best character), and if you've very confident, go ahead and turn on Hurry and Hyper to really speed this up. Utilize your best weapons/evolutions, such as Santa Water, King Bible, Axes, Spinach, etc., and you should be able to hit level 80 around 12 or so minutes into the level. Simply quit out after hitting level 80 to end the run and unlock this achievement, as well as the new stage, Bat Country.