I agree with you on Hiruzen, and Mei is definitely underrated. I don't see how Tsunade beats an opponent like Ay, though.
Ae has to charge his armour to the max. It's not something that is instant. This time-lapse is exploitable. If her immortality seal is active, she should be quite durable for his punches. Katsuya can be summoned as well to divide and conquer. One of them can be latched to herself to heal if she receives damage.
Recall Karin's statement. Ae's Chakra had risen to Bijuu level when he upped to V2 armour. He has a large chakra pool that much is understandable, but Christ Almighty, there is no way in ****ing hell he would be able to maintain that for long. People forget THIS important factor: His V2 Cloak requires a hell lot of juice.
Tsunade can simple hide within Katsuya as well to stall him. I definitely believe Tsunade takes him.