Yeah, this was just me wanting something OP in game xD also it fits the mythology of Ambrosia granting immortality, but having another +% Wizard HP seems redundant.
To make it less over-powered, it could be a 2 item set. With potions, the effect can be spread to many towers and the effect can be doubled or tripled using Yggdrasil and wedding rings. With an item set, one or more items can be unique and then it can be limited to a single tower. Andy, what do you think about a 2 or 3 item set that let's max level increase by 5-25? I estimate each level would give a damage bonus of about 2% on top of the normal base damage growth. Even 5 levels would be a significant damage increase. Whatever the increase is, it should be balanced against the 40% given by hydra arrow, a single item. I imagine the set allowing the xp to grant additional levels. So it doesn't change xp or levels, just the level cap.