Here is the full list of all 20 The Legend of Gwen achievements.
500 Coins
A rare Coin
All Chests
All Stars
Beat The final Boss
Unlock CheckPoints
Complete all LVL 4
Die 30 Times
Find all Seeds
Get all Skull medals
Help Corki 6 times
Help Corki once
Unlock a New WAND
Use the Boss Defeater
World01 Complete BOSS
World02 Complete BOSS
World03 Complete BOSS
World04 Complete BOSS
World05 Complete BOSS
World06 Complete BOSS