This Achievement is unlocked by defeating the final boss and going through the post-battle dialogue sections to trigger the endgame sequence and credits.
For tips on the final dungeon and boss battle itself check out the solutions for the same Achievement in
Earthlock: Festival of Magic. The boss battle strategies are similar although the extra weapons you obtain in
Earthlock may make the battle slightly easier due to the stat buffs you can gain.
One extra tip that I can give is to use PAT's Hover ability in the final dungeon. This will allow you to move quickly and potentially avoid some tough battles before the final boss itself.