I was able to do a full clear on my rogue the 3rd time (1st time being the story quest, then the WQ for times 2 and 3). When facing Dro, don't be afraid to do your agro drop and let the withered tank while you heal up. Always bring at least the 1300 ancient mana level of withered with you. 2k is great if you've bothered to unlock the cap that far, but it's overkill if you have a couple upgrades (damage and hp) and are picking up the withered from each room (not the first 2 berserker required room!) as you go.
if you started with 16 using 1300 ancient mana, by the time you get to Dro you'll have 25+ in your army. Berserk them and stun him when he starts zipping around. The rest of the bosses are a joke or are skippable. Dro, the final spider boss, and the room with all the eggs are the only REQUIRED encounters to finish the instance (which in itself isn't required to get a 425 score - 425 being the max rep you can receive). Literally every other boss is optional. The Elfbreaker definitely requires some patience and care to move your way around the room without agroing him, so I wouldn't recommend relying on doing that correctly the first time you try. The 25% damage is in his room behind the knocked over pillar. Bring at least 5 withered with you and kamikaze to it if you think you're gonna lose them soon or you accidentally agro him.
Also, I noticed Ret pally's Blessing of Might extends to the withered. It's a pretty huge advantage over other specs, so I'd be surprised if it doesn't get nerfed.