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I always thought that the mob had to use the ability on you. If you're tanking, and it is a single-target ability, that works. If it is AE, then you don't need to be tanking.

I learned at least some of the abilities for all the classes. Unfortunately, I can't remember the details of most of them.

A lot of them required you get hit by them. Not always though. My cleric started learning an ability (I think it was ward shield?) from a mob that was inside the walls on the ground level of Vol Tuniel, while I was killing the mobs on the outside.

From what I remember, your character simply needs to identify/detect them using the ability.
You most likely need to be involved in the fight obviously

Just try not to counter/or interrupt them doing so