i went and took vampire to increase fire damage i took so i could better test it
i finally made it proc one against one guy's meteor that i took head on for maximum damage, though i died the next second
it seems to work, but the damage has to be direct. it does not work on cumulated dot ticks, and does not seem to work when the damage is absorbed in shields or blocked.
I'd consider this CP passive as one of the few "must have CP" passives for glass cannon builds in PvP given the actually great numbers of sorcerers (shock damage, sometimes fire) and Meteors (frost damage). Especially good for glass cannon builds where 30% HP is around 600 dmg.
But we'll soon have much more 2,5k HP+ specs so it's utility in PvP is low. Especially bow damage hurts.. and that's not a seldomly picked weapon..
I think it is pretty useless for a passive as Most glass cannon or pure magicka sorcs don't have magicka problems.
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but not everyone is a glass canon or pure magicka sorc.
there are others, for example the ones that receive the damage of those two builds i guess those would like some form of advantage of receiving those very high crits.