Here is the full list of all 21 KAWAII SLIME ARENA achievements.
Complete 15 zones
Unlock all weapons
Complete 10 zones
Have 3 different bats at the same time
Have 3 different ghosts at the same time
Complete 5 zones
Unlock 3 weapons
Deal 10000 damage in total
Defeat 100 waves
Unlock a new weapon
Defeat 1000 slimes
Shock a slime
Upgrade a weapon
Complete the first zone
Deal 1000 damage in total
Defeat 100 slimes
Deal 500 damage in total
Complete a zone at full health
Poison a slime
Freeze a slime
Defeat a wave