Mobs with trash right before boss have a buff they put on themselves. It increases damage by around 10%. You can then kill them until a mage spell steals 8-10 stacks. When the mage is at max stacks, kill the boss and let the mage remain within the blue beam for the entire phase. Give him all the innervates, mana tides and healing you can get while being able to heal the mage. These numbers for frost and fire are achievable with Arcan approximately 5k.
My friend, who is a tank and healer as well as me, run heroics. We'll carry the dps as long as the boss can be hit (level of 68-level the 73 boss). There are dungeons in which it's possible to have an issue, but there are plenty of heroics where it's not a big deal.
It happened to us a few times and we were able to salvage the run. It was not one of our belts however it was a fantastic save. As long as you've handled the adds and are in healthy health of your shield, you're typically safe. But if you've been slack and let adds slide, it's very difficult to salvage.
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