Doom is the best counter for gwenperion
Dont Believe me? my r2 6* solod her in 138 hits.
Tactic is as follow. you spam heavy only. nothing else. just as the timer is about to expire for the regen. you parry and 1 medium and dash back. wait the shock out. parry heavy strat again. go for sp3 only as mine did 3-4% per sp3.
It requires EXACT timing or you fail. I messed up and she regened to full hp but I got her down to 0% again after getting the timing done.