I have a 650 ilvl monk and a 645 ilvl warrior. Both are tanks and they have around the same quality of gear.
My warrior steamroll tanks all bosses in mythic, but the first boss in Auchindon basicly 2 shots my monk. I just cant think of anything i can do that i'm not doing and i stil lget absolutely pummeled by that boss.
I start out with fort brew and guard up. Guard + 10% hp is gone in 1 hit. Second guard is gone next hit. After that i survive a few seconds on Fort brew but even that is jsut not enough to keep me going.