Oh, this had so much potential to be an awesome mash-up!  You can "Luigi-fy" Snake's uniform or vice versa, but as it stands right now, it's too much on the Luigi side to get the reference to Metal Gear Solid.

this has to be the least metal gear thing i have seen.

 Just throwing this out there but, instead of making him look like Snake, I'd suggest pixeling him in Yoji Shinkawa's style. Also, though your shading style is ... unique, and does do a good job of showing off light and shadow, I'd suggest using less colors and go for a more solid shading style (to fit the metal gear feel).

I know, in fact I would Take Luigi and Draw him with Human proportions, and the Challenge want two caracters but I have melanged Luigi with a buff human, because it was what i want to do, and "Metal Gear" because Snake look like this human. I Know It's too much but it's juste because of the Gun and the skull. I could name it Luigi and James Bond lol

 ( sorry for english )

I really don't see the Metal Gear influence in this. Just looks like a buff Luigi with a gun. The only Metal Gear influence I see is the Question mark and Exclamation on the enemy's head. Everything else doesn't really mesh with Metal Gear style...Also, too much dithering is bad for the soul...