Here is the full list of all 40 Guilt Battle Arena achievements.
Complete the tutorial
Win 1 match in versus
Defeat the dreadful Judge
Get 3 stars in First Rodeo
Get 3 stars in Exploding Cats Factory
Steal 20 bullets as a ghost
Explode 50 cats in versus
Touch 10 players with a bomb explosion
Stun players 50 times
Pass the rocket target 50 times
Win a round in versus with Trampoline and all your hearts (3 or 4 players)
Win a round in versus with Invisible and all your hearts (3 or 4 players)
Get 3 stars in Cannon Fodder
Get 3 stars in Rocket Science
Win a round in versus without shooting any bullet and all your hearts (3 or 4 players)
Win a round in Capture The Flag without losing any flag (3 or 4 players)
Win a round in Predator without losing the predator mask once (3 or 4 players)
Win a round in Last Man Standing and all your hearts (3 or 4 players)
Play 100 rounds in versus
Destroy 500 enemies in co-op
Get 3 stars in Holy Chicken!
Get 3 stars in Deadly Coliseum
Finish a match with 100% accuracy (3 or 4 players and 3 victories minimum)
Win all the rounds in a versus match without losing any hearts (3 or 4 players and 3 victories minimum)
Play 500 rounds in versus
Destroy 50 enemies in a row without getting hit in co-op
Play 1000 rounds in versus
Win at least 1 bronze medal in each cup
Win a round in Third Eye without getting shot by a laser
Steal 50 hearts in Heart Stealer
Get 3 stars in Close Encounter
Get 3 stars in Blobs From Outer Space
Win at least 1 silver medal in each cup
Win a round in Parasites without getting damaged by a parasite
Win a round in Fatal Dodgeball without getting hit by a ball
Get 3 stars in Dodgy Stadium
Win at least 1 gold medal in each cup
Win a round in Alien Pet without losing the alien once
Win a round in Gold Rush with all the gold
Get 3 stars in Backward Ship