Bubble pulling is another method to use if you have too many casters and don't have any corners to LoS nearby. Examples are the 6 caster on the last boss of Blood Furnace, the circle room just before the 3rd boss of Magisters, and my favourite, the room there the hidden bookshelf that leads to Illhoof in Karazhan.
The way bubble pulling works, is if the mob's aggro tables have only your threat and no one else, they'll all run towards you you melee you. Reason is bubbles are immune to all forms of damage, especially casting so they all just run in and melee you. The most important thing of note is that the aggro tables
must not have any threat at all of anyone else, and if anyone else builds any threat they'll make a beeline towards him/her instead of you.
It's tricky to use until you get the hang of it, so practice using it whenever you can.
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