From what I was hearing leading up to this banner, just NP1 is fine for Asclepius to work. More NP levels gives more HP for the Guts revival, so that's pretty negligible. Overcharge grants more HP recovery per turn, but considering it's a healing NP I feel like it would get low priority in an NP chain, at least imo. Main thing is getting the Eggs to upgrade his skills once you hit 8 on them.