Here is the full list of all 25 STAB STAB STAB! achievements.
Watch 500 fleshbirds pop
Die to ceiling Spikes
Die to the needles in the hypodermic stage
Clear 20 waves of cronenbirds in survival mode
Clear 10 waves of cronenbirds in survival mode
Have more than 6 legs at once
Defeat a cronenbird in versus mode
Fall into a meat grinder
Win 10 versus matches
Watch 100 fleshbirds pop
Get squashed by a falling holo-cube
Get a kill while you have no legs
Get popped by a rolling spike ball
Jump through a teleporter
Pick up the wings and beak gun powerup at the same time
Watch 10 fleshbirds pop
Win a versus match
Die to a swinging hammer
Block a spike with a tumor
Pick up a glob of radiation
Win 100 versus matches
Finish 20 versus matches in a draw
Watch 1000 fleshbirds pop
Watch 5000 fleshbirds pop
Clear 50 waves of cronenbirds in survival mode