I've tanked heroic MgT probably 12 or so times, and my experience has always been the same: if I bubble while he's casting pyroblast, it will hit someone else. However, it's fairly easy to just start strafing away from him the moment he begins casting it, and then bubble after it leaves his hands. That has worked for me every time. Forewarn your dps that they should interrupt his next fireball after the pyro, as that will most likely be aimed at someone else.
Also, is he immune to taunt? I thought I got the immune message the last time I tried to taunt him.
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