He has the opportunity to melee if all spell schools are locked out, which happens when his fireball, frostbolt, and arcane missiles are all interrupted one after another in a short period of time. He hits for something like 500-5000 depending on the armor of the target, which can sometimes be bad when the target is a rogue a squishy warlock or mage. He will also sometimes move out of the center to chase the highest on agro at this point to get into melee range to hit them at least once.
I have seen this melee attack kill someone, but this was many months ago with a bunch of undergeared people learning the fight.
Anyways, I suggest that you make sure that, if you are healing and there is no "tank" so to speak, that no one interrupts his arcane missiles, or you risk someone taking a melee hit which can sometimes be bad.
If no one interrupts the missiles he will stay in the center and not melee anyone and it is definitely better to help heal at this point than worry about doing damage or remaining top threat in the case of a melee attack. It is almost impossible to remain top threat because you will run out of mana very quickly.
It also makes sense to heal because the damage output of this fight during a progression attempt is greater than normal and disproportionate with the damage output on the raid for most of the other parts of the instance. Take advantage of being a hybrid class here, don't worry about tanking, and if he does end up meleeing and killing someone, blame the rogue or warrior or whoever interrupted missiles.
This is a good practice anyways as interrupt abilities are used throughout the rest of your raiding career and its a good idea to make sure the interrupters eventually stop sucking.
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