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