So my guess is this:
Kasia intentionally take a blow and pretend to be down just so this guy will get closer, then go for the kill when he's close enough (possibly using whatever the mana thing she did to cut the ring of trees shown while training with Ernon earlier)
Then regardless of whether she manages to finish this guy off, the prince's army will arrive in time to save her, since she stated that she's heading for them earlier, and it'd be hard for the prince's group to ignore the constant explosions.