Your Method and team comp is exactly like my group. Here is some suggestion:

Get your drg to skull sunder the rook and he will never lose aggro.

I don't know the tank or healer rotation but all I know is we always LB when MT & OT has everything pick up, one of them will TANK LB

Our kill order:
1. Everyone on rook and myself(drg) tank it. Skull sunder to keep aggro and I pop mantra to help the MT as well even it's 5% it help.
2. Once rook is dead, we kill the old dreadnuaght.
3. When old dreadnaught is dead, we have MT pull to the OT to feed the 2nd Dreadnaught the spiders.
4. We burn down the solider and knight then last dreadnaught.

We had turn 4 down when most of us was in full DL and relic, we had to use food and pots back then tho to get it down.