Here is the full list of all 31 Peglin achievements.
Inflict 50+ stacks of poison onto a single enemy
Defeat Betsy the Leshy
End a battle with 100 Ballwark
Complete Cruciball Level 10
Die during an event scenario
Bramble an enemy for six turns in a row
Defeat any boss on the first turn
Hit 1000 pegs or more in one shot
Fully clear a pegboard
Deal exactly one damage
Defeat the Slime Boss
Defeat the Animated Ruins
Deal 9000 or more damage in one shot
Defeat the Ballista Boss
Defeat Avogadro's Tree before entering the Castle
Defeat the Wall Boss
Leave an enemy with just one health point
Defeat the Mole Boss
Deal just enough damage to kill an enemy
Complete Cruciball Level 15
Get three Bramballs stuck in Leshy's vines at the same time
Win a run with two eggs in your satchel