This is a crossover challenge between the Halo universe and Shin Megami Tensei.
It would be prefered if Halo: Reach or Red vs Blue were involved.
Master Chief is a badass but I don't know, too much OP if you think about it, that's why there is the second best badass Spartan ever existed, B-312, a.k.a. Noble Six, due to him/her tragically losing their team, and finally their life. Being transported to a demon infested to a modern day demon-infested Tokyo could be a second chance in life for redemption. Also, humanity in their world is in a war against religiously-extremist-non-humans, so, for want of a nail… let's see if a Spartan can best TRUE Demons and rob them of their title.
Or you could do a comedy with it's serious moments like with Red vs Blue in the world of SMT IV: Apocalypse. That game focuses more on Neutral positions and relationships, not to the extent of Persona, but still, the two are more of grey and grey moralities. Due to a freak accident, the Reds and Blues (with Carolina and Wash) end up in Underground in a four-way war between the forces of Chaous, Order, the Divine Powers and humanity. When they cross their ways with Nanashi's party, what do you expect them to do? The best they do: wreak havoc to everything! Observe as Lucifer, Merkabah and Krishna's plans fall apart due to the most eccentric bunch of humans that they've ever seen, and Dagda enjoying their suffering. The thing is, having the most idiotic soldiers in all the known universe in an almighty cosmic entity's scenario, the result will not be quite as expected…
One thing's sure, the MJOLNIR will serve also as a COMP, in a fashion similar to the Demonica from Shin Megami Tensei: Strange Journey. The sim trooper and Freelancer armor, while not as advanced as a Spartan's, they have 26th century computer technology, so they can also run the Demon Summoning Program. And if used alongside an AI, the user's efficiency will increase.
Show the mighty and/or randomness of humanity while kicking demon asses, to the point of having the Doomguy feel proud!
You could also put Alpha Squad from Halo 3: ODST with their armors also as pseudo-Demonicas.
