I was going to say that a legion of reaper-ized flood, covenant, and ceph supported by the reapers themselves might be the only force that could give this legion of heroes pause, but then I saw Ryu Hayabusa and realized that every force these guys have ever faced is fucked through the ass out the ear with him alone. hell if Raiden from Metal Gear Rising was in there then everyone else can just kick back and take a break.
Have you thought of doing a piece that would have some of the other heroes at a bar, drinking and moping about this? it could go something like this:
Shepard: "Ninjas? how the hell are we suppose to fight with a guy that can bring a sword to every kind of fight and win?!"
Prophet: "Oh I don't know, how can we fight with a guy that can burn shit with an orange hologram, blow shit up with your brain as well as a huge selection of guns, five of which, you can have on your person at one time?"
Shepard: "Hey those have some cooldown times and I have my limits, what limits does that guy have? I've seen him take hits that would have killed all of us."
Master Chief: "I know, I saw him take direct hit from a rocket launcher, a freaking rocket launcher! That hindered him for less than a second, then he was back at it like all they did was punch him"
Shepard: "how many hits like that can you guys take?"
Master Chief: "None!"
Prophet: "same here, wait, how many can you take!?"
Shepard: "Well... the rockets of my world might be different than--"
Prophet: "You can take several can't you!?"
Shepard: "...... I should go,"