Ern was one who was promoted for his organizational and tactical skill, and ultimately being able to keep a team together. But that wasn't to say that he had significantly lesser skill in combat. He typically did depend on both physical strength than gross motor speed, but could quickly turn a good spell with what and who was there. He wasn't one to completely doubt the skill of his opponent, even if he had some sense that they may be impaired. And beyond being a gentleman he wasn't one to let any female off any easier than his male colleagues--it wouldn't be fair to them to have a lessened training opportunity, after all.
He did dodge away from her lunge, turning to drew his wand from the splash of water. He aimed a spell at a punching bag on the corner closest to that left hand side from where he drew his wand and sending it through the air towards where he had anticipated her movements to take her.