Skyforge

You'll probably pick this up in the process of doing something else considering how long of a completion this game is, but if you want an easy way, try this. Go to a campaign mission that's way below your level (I did Alakur Island, but most missions will work). Run past all the enemies to the first boss. All the regular enemies you pass by will follow you to the boss room. From here just use whatever AoE attacks your class has to wipe them out to easily get this. The Cryomancer's X attack works well for this, and the Lightbinder's RB attack works too. Not sure about the Paladin, though.