I hope there's nothing crazy going on with this training mode, because if not, I think I've found a way to get Zack to CB in 3 hits.
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1. I'm assuming this is because Zack has to play the stun game, and also the move can be countered mid not matter if its a punch counter or kick counter. Also you have to bait the counter for it to work but,
DUK P -> bait the counter -> P+K
from there you use either CB. Could the P+K catching the counter in recovery be causing some type of revision on the critical damage?