Tainted Grail: Conquest

Treasure Guardians are powerful monsters that protect a treasure hoard (looks like a chest or wagon), which will give you 100 Wealth and a pick 1 of 3 bonus, that can be Armour, Wyrdshards, (temporary) more Runestone slots etc. If you mouse over a monster it will be labelled with what kind of encounter it is, with Treasure Guardians having their own label. Successfully beat the combat encounter for your achievement. If you are looking for the easiest class to play, I would recommned the Summoner, with the Golem Spawn Passive Skill.