Here is the full list of all 15 Mighty Mage achievements.
Win a game as a Fire Mage
Win a game as a Water Mage
Win a game as a Lightning Mage
Win a game as a Earth Mage
Kill 100 Slimes
Kill 100 Bats
Kill 50 Fire Mage
Kill 50 Water Mages
Kill 50 Lightning Mages
Kill 50 Earth Mages
Heal more than 100 points at once
Cause a critical hit with more than 200% additional critical damage
See credits
Kill 5 enemies with a single spell
Win a game without taking damage