This boss comes after you have beaten the shadow created by a Black chronicle servant and obtained the beast mark.
Please note that this boss has no weakness and can deal successive high physical damage.Taking up all 9 squares of space, Aht isn't advisable as a unit since you won't be able to move the enemy into traps you set up. Based off of the damage taken from both a physical and magic based attack, it appears that the Guardian has slightly lower resistance than defense so strong magic users like Raynie should be on your party.
Moves to note are Deadly: dealing damage that can poisonous.