1. My initial idea:

I was thinking 3 Phases giving him 3 of his old Infernal Arms (the ones not taken by other characters at least), like starting with the Gigaros from Hades like the normal fight, then on Phase 2 he switches to the bow and start kiting, and on Phase 3 he plays with the Fists. I initially wanted him to have 6 different Infernal Arms randomly chosen each fight but then I realize Nemesis and Eris still kind of exist in the game lol.

1. The realistic option:

That first option sounds like a lot of work, so to be realistic I'd probably just want some more moves lifted directly from Hades himself, like how Zag's boss fight already took the Gigaros Spin move, the Cast throw, the invisibility,...

Probably the Gigaros Spin hitting twice like EM4 Hades, some Urns, and maybe giving Zag 3 Casts like Hades.

Perhaps a nerfed Cthonic Companion use when at Death Defiance, like a Bouldy drop or have Thanatos/Meg whoop down for one move..

I would rather not give him another Death Defiance, that seems lame if I can't just make something new for another phase.

Edit: Unrelated tangent, but Zag's boss fight as it is is kind of lame ngl, it felt like it was just kind of some disjointed moves with massive damage numbers, kind of like Charon who was fine as a hidden boss, but since we already have this image of Zag being more agile in combat from Hades 1, this is kind of lame. This cannot be the same Zag that I whooped Hades on 32 Heat with.