Nihal

Well I believe it is said that he lost his herrescher powers(power to create anything which he fully understands) because the new herrescher of reason is Bronya.

That being said tho he still has the 2nd most broken weapon in HI universe aka the star of Eden.

We still don't know the full extent of acheron powers but no way can she go against a physical phenomenon such as gravity.

Star of Eden doesn't just manipulate gravity it can do it in such a way that it bends the laws of this universe. Bro can create synthetic black holes, near unbreakable shields and singularity(a projectile with infinite mass and is unblockable).

I would say welt is slightly above acheron because of his vast knowledge and battle experience. If he had his herrescher powers the fight wouldn't even be close. Welt with herrescher powers is nearly as strong as an aeon.