I don't know any math behind this, but if you check out the officials Archer forum there's a thread with archers' builds and skill empowerments etc. on it.
One 8x archer has 31+% crit (w/ CS)
I can see that archer wiping out someone with pure crits. xD
Then again, criticals that double your damage when you don't have much damage to begin with are rather useless.. /shrug
-doesn't want to do math-
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