Here is the full list of all 14 Guardians of Graxia achievements.
Hit at least three enemy units simultaneously with either the Hell Storm or Wild Fire spell cards
Eliminate a unit during the Battle Phase by sacrificing one of your own (for Map Pack 1)
Win 5 Campaign maps with Guardian Victories
Raise a single unit's Attack to 25 (excluding Large Creatures)
Raise a single unit's Defense to 25 (excluding Large Creatures)
Raise a single unit's Health to 25 (excluding Large Creatures)
Win 5 Campaign maps with Standard Victories
Win 4 Skirmish maps with Guardian Victories
Win a map in 1 turn
Beat the Campaign
Eliminate an enemy Guardian with your own Guardian without dying (for Map Pack 1)
Achieve a Guardian Victory on any Skirmish map with all available Guardians (for Map Pack 1)
Eliminate 15 enemy units with 1 Large Creature (for Map Pack 1)
Cast the "Gift of Efficacy" spell on a friendly unit