Dude this is EXACTLY what happened to me with Geshniz. she was 1-shotting me and it was happening exactly like you said! you might find useful the way i was able to solve my problem.
Yeah, I found pretty useful way called "cheating" since I've restarted that battle gazillion times and was totally pissed
Now I'm sure, that it's a bug, not a disbalance, due to lots of complaints.
