Does Master Chief have balls of steel?
No, he's just covered in steel. Duke doesn't need steel armor. HE HAS PURE MUSCLES. (and balls of steel)
Besides, Master Chief has never literally ripped someone's head of and defacated down their neck.
(skip to 0:19)
Yeah, I think Halo is a pretty cool guy, eh killz aleens and doesn't afraid of anything, but Duke somehow nukes almost every level in the game and survives the blast.
If you can somehow nuke yourself and survive without thinking about it, you truly are afraid of nothing.
I would suggest a team of the 2 saving the world, but MC's "Get the job done" motives would contradict Duke's "Get the job done and have loads of fun doing it, spewing one liners as their limbs fly away" ideals on saving the world. Maybe MC and Doomguy on one half of earth and Duke on the other. That would work out, wouldn't it?