Tainted Grail: Conquest

Treasure Guardians are tough monsters that protect a treasure chest. On the map, they will appear with the chest sticking out of the ground behind the creature you must defeat, (which has the label Treasure Guardian) rewarding you with 100 Wealth and 1 of 3 Random Bonuses, which can be Max HP, Armour, an extra Item or Runestone Slot, or Wyrdshards. There is only a single guardian per zone, so you must kill all 3 in one run to get this achievement. Which monsters you face will vary, though the hardest version by far is the one with the silver wyrm creatures, as they throw out a lot of Armour reducing attacks with heavy damage.

In my opinion, the easiest class to complete this with is the Summoner. Summon a Golem to absorb the damage and a Wyrm to deal damage to the enemy. If you have the Silver Wyrm variant, an Abomination may also be necessary to deal with the smaller wyrms. Buff the Wyrm's damage Armour reduction and Promotions as much as you can, though make sure to also keep a Barrier around the Summoner and the Golem in good shape/summon a new one, otherwise it can go very wrong, very fast. The more you promote and buff your Wyrm/Abomination, the less time they have to kill yor Golem and you.