You don't list your team makeup, but you might need to take a look at what toons you are putting up on the weak spot. I was trying to do it with my Enh shammy and it turns out there is some sort of bug where autoattacks from Enh shammies won't hit the Weak Spot, only specials will, so she was sitting up there doing no damage and burning all her mana while the rest of my team got wasted. I didn't figure it out until I noticed she was totally out of mana and that simply doesn't happen on Enh. I stuck my UH DK up there instead and steamrolled the fight first time out.

Also, I leave my slave on the boss while he charges. When he hits the wall, he throws the slave up on a little ledge next to the gate, directly above a cannon, so I can simply click the cannon and CTM will drop her off the edge, into the cannon and up on to the boss in one move.