Here is the full list of all 20 Copperbell achievements.
Finish Grub Cave without getting hit
Finish Cave without getting hit
Finish Rotten Tree without getting hit
Finish game without getting hit
Finish game without dying
Finish game without any upgrades
Buy all upgrades
Die 10 times on the first level
Open all gates in Cave
Open all shortcuts in Cave
Complete all jumping challenges in Rotten Tree
Die 5 times on a boss
Die 10 times on a boss
Die 25 times on a boss
Clear all mosquitoes in Rotten tree
Clear every enemy in Cave
Clear all enemies in Grub Cave
Get crushed by a spider shell in Cave
Looked at everything
Find all the gold