Here is the full list of all 12 Shoot Many Robots achievements.
Destroy a robot overlord in multiplayer!
Kill 100 robots!
Revive other players 8 times!
Play co-op!
Beat a mission wearing the astronaut helmet and backpack!
Kill 10,000 robots!
Reach level 50!
Get 75 stars!
Beat a mission wearing the baby carrier!
Complete a mission on hard or insane without dying or drinking a beer!
Punch two bullets, killing two robots, with one melee swing!
Come in 1st in a four-player mission!