Whoever said that OT shouldn't be tanking every trash mob in Phase 4 is not true. He should be tanking everything and he should be dodging Pox from the Rooks by running.

If you are not killing the Phase 3 Dreadnaught within 10 seconds into Phase 4, you guys have a DPS problem (most likely your dps not dpsing optimally). Try using food and potions or make it mandatory for your DPS to use. Your group's equipment is not bad at all, but a few ilvl 90s should help.