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