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