So I've completed Agatha twice now. Once with my unholy DK and once with my fury Warrior.
The DK was a bit of a pain but I managed to chug through with a 4-piece LFR set and just crushing burst damage. Not even with the shoulders that empower your army and gargoyle/arbiter, but fast ticking diseases and BOOM!
My warrior though - even having better gear and I believe a few of the T2 crucible traits unlocked (it's been a few weeks) just couldn't push the deeps. I'd swap around talents and change trinkets and gear but it just didn't want to push. I'd try saving Battlecry with Odyn's for when the umbrals would spawn and nothing seemed to really work.
Then I read you can use the potion of demonslaying. What's more is that particular potion doesn't count against any of your other potions - if I remember right. So you could have it, food, flask and pre-pot all up at the same time. It still took a bit for everything to line up properly but the potion was my #2 damage source (it is absurdly good) and I finally got the ugliest transmog skin in the game.
The trick only works on Agatha - sorry tanks, I wish it worked for Kruul too.
Anyone else push your tower with this method?