Video courtesy legend of fighting game tech discovery Dr. Adaki
you can actually put saitama on your team, it's not restricted to a fake saitama or a saitama stand-in. but if you do, you'll need to play a character/assist down, and saitama will only come in after about 3 or so minutes of waiting around. you can speed up this timer by being good at the game (anything above "basic" combos, perfect guards, etc), and when he arrives to the fight you've effectively secured your win condition
so, effectively, you have to play a 2v3 for at most 3 or 4 minutes until you wait out the timer and get saitama on your team. which means you just win
of course, you can play as pyjama saitama, which gives him a real moveset at the expense of making him around-60 punch man instead of one punch man