Here is the full list of all 14 Guardians of Graxia achievements.
Cast the "Gift of Efficacy" spell on a friendly unit
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)
Eliminate a unit during the Battle Phase by sacrificing one of your own (for Map Pack 1)
Hit at least three enemy units simultaneously with either the Hell Storm or Wild Fire spell cards