Here is the full list of all 23 Footgun: Underground achievements.
Get a score of 15000 in the training room
Defeat the Large Exploder
Defeat the Security Eye
Start your 10th attempt
Defeat the Giant Divider Slime
Defeat the Big Nut
Survive a Rage Boss encounter with exactly 1 health remaining
Defeat the Myriapoda
Defeat the Spewing Slime
Complete an entire Biotope without getting hit once
Crack open the Pinata
Successfully complete the Target challenge
Defeat any boss in under 120 seconds
Defeat the King Beetle
Acquire a new item from the Gum Machine
Keep a ball in Damage Mode for at least 10 seconds
Defeat the Fortress
Defeat the Mutated Plant Mass
Defeat the Laser Defense System
Defeat each Boss
Complete your first Full run
Empty out the Gum Machine
Dodge 3 times in a row