NHF – new heroes for Heroes V 3.1.

This way, you can easily level this hero up to 40 lvl, because he won't lose anything

No, but you were talking about "enemy heroes," not neutral stacks.
Although for the sake of fairness, adding neutrals to this scheme doesn't make it OP, but there's little point either. The reason is simple: tempo. There's no sense in trying to attack the same neutral stack several times and gradually reducing its numbers: it still takes a few days to reach it. And while you are tormenting a neutral, the draugr levels up the opponent levels up.
I believe that complications with reducing experience and stats aren't needed at all. Just figure out the respawn time and location, and it will be easy to balance.