Newgrounds.com — Everything, By Everyone.

I beat him. The big boss in the black and red armor.
It's gonna take a while though.

Follow these steps and you'll do it.

1. Pick Sanford. I know he doesn't look like much, but just PICK SANFORD FOR YOUR SAKE. His hook is melee AND ranged.
Plus, he doesn't let go of it when punched by the boss.

2. When the Boss has his Axe, keep your distance and keep throwing the hook at his FACE. He'll let go of the axe eventually and you'll notice his mask is cracking.

3. This is the tricky part, be careful. When he uses his minigun(the big gun that looks like a gatling gun), stay away from the bullets. Deimos will probably be killing all those henchman who come near you. If not, than kill a few yourself. Keep throwing the hook at his CHEST.

4. He will eventually get knocked down and lose the minigun.
Deimos will probably be dead by now, but whatever... You can beat the boss alone at this point. I did. Now, keep throwing the hook at his FACE AND/OR CHEST. He will keep chasing you and trying to punch you, but just keep your distance.

5. His mask WILL eventually come off. And I really don't need to tell you what to do next...

Hope this helps!
n_n