So lets say he got off the first chaos bolt had 6 stacks of backdraft, first cast takes 2 sec, you obviously didn't notice this because you were cc'd or terrible, or a combination. So the first one goes off, 2nd one 1.75 sec cast with backdrafts, then we have a 3rd one where your stupid teammate, or you dispelled an immolate during wlock burst. it casts at roughly 1.2 seconds, now we have approximately 3 seconds between the first cast going off until the 3rd cast going off. Idk how people manage to do the math that it takes 7.5 seconds. lets say he has roughly 10% haste too, making it <3 seconds
in the first place he got off 2 1.75 sec chaos bolts in a row and that is your fault, then let him get off a quicker one due to dispelling rather than healing.
Its the best possible outcome for him to get off 3 chaos bolts, the stars have to align perfectly though, so while it is highly unlikely that this is what happened it is possible.
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any decent player is already using a damage meter for this though, thus you already have a timer.