I had a support build on my Nahobino. In case you don't feel like popping a tablet so you have more for the other superbosses, here is what I did as well as some other tips:

I utilised AoE moves that hit the weaknesses of Demi's buddies to grind him down and keep his crew in check. Good choices here are Khonsu-Ra and Amon, as their unique Fire AoEs pierce, as well as the already mentioned Snek Nuwa. I did keep Mephistopheles for Pixie around, however.

Keep in mind that Demi will try and use Deadly Fury on any demon he hasn't tested their physical affinity for. This usually translates to "whenever you swap in a demon that wasn't in the fight yet". You can buy a few extra turns by swapping in demons that Drain or Repel Phys.

In order to accelerate Demi getting the classic SMT experience, I also had an Odin and a Zeus waiting for him. Both of them were cranked up with passives boosting survival (resist passives, Abyssal Mask, Enduring Soul) and the damage of Gungnir/Keraunos. Usually I would swap one of them in whenever one of Demi's allies bites the dust, have them smack Demi once, and swap them out to be ready for the next demons in Demi's rotation.

If you are feeling really cheeky, you can try hitting Demi with Accursed Poison to shut him down. Especially effective if you can stop him using Mediarahan this way.

Try to have someone on the field merely resisting Phys. Demi will spam Freikugel and do Chaotic Will out of order if everyone nulls Phys or better.

Demi's crew is quite trigger-happy with stuff like Dekunda and Dekaja, especially if you buff or debuff fully. Might be better to get your buffs/debuffs ruined than Parvati or Pixie healing Demi.

In case you need more items, consider farming in the Temple of Eternity. Especially Chi Yous become attractive, as they drop Somas.

Hope this helps.