I dunno if you can change the feats while you have a checkpoint but I would suggest this... Farm final boss checkpoint with (Rez tokens, Battalions, Phase Race and challenges on) If you can change feats with a checkpoint just get to final boss normally and just save the checkpoint on another char just remember to launch it with the feats active. You can complete the challenge very easily by just repeatedly killing the apprentice and shooting all the orbs in the normal realm until it none remain then it takes you to the taken realm where you will immediately compel the middle guy and shoot an orb. (challenge is to shoot an orb within 15 seconds of each teleport but you don't need to really do that if you never teleport more than once) You now have infinite time in the taken room to do the mechanics normally before damage phase and collect ammo from the thrall in middle. Just repeat if you need a second damage phase and then just keep repeating the boss for T5 loot. Final boss drops all weapons afaik.