Here is the full list of all 20 God Mode achievements worth 400 gamerscore. It takes around 25-30 hours to unlock all of the achievements on Xbox 360.
Purchase all upgrades
Reach the final level and attain God Mode
Grind 50 creatures using the Buzzsaw melee attack
Kill 20 creatures using the Hammer ability
Complete all maps on hardest difficulty
Finish any map with 6 active Oaths on hardest difficulty
Finish any map with 3 or more active Oaths
Enrage 10 Cyclops
Fully upgrade one weapon
Kill 2500 creatures
Destroy 50 scarab eggs before they hatch
Decapitate 100 Skeletons
Complete any map without dying
Complete your first Oath
Survive the "Death" Test of Faith
Finish any map on any difficulty
Purchase your first weapon upgrade
Trigger 20 Mage seals
Kill 50 enemies using melee
Kill 100 creatures