The first thing that strikes me about your version of the poster is how very awkward looking the figure's front leg is. That is to say, because of how flat the guy looks and how little that front leg is defined, it looks like he has a foot growing out of his pelvis. The lack of depth and definition can be quickly fixed by making the everything but the front leg darker. It'd also be a good idea to change the black shadows to a dark shade of yellow. The black bits contrast so much with the rest of the dude that they really don't look like shadows, especially around the legs.
Stylistically, it's kind of all over the place. The guy in the suit and what he's standing on are super simple compared to the realistic smoke on the left side of of the poster, and the UFOs are somewhere in between. This causes a bit of confusion when you look at the picture; the UFOs are more detailed than the hazmat suit dude, so your brain interprets them as being closer. Which, if I'm honest, makes the poster at least ten times more awesome. They almost look like they belong on the set of some old sci-fi B-movie involving aliens fighting a giant.
It's a good start, it just needs some more work. For one thing, you should probably get rid of the smoke. It just doesn't match the rest of the poster, and it's more of a point of emphasis than what should be the focal point: the hazmat guy. That smoke also looks like it's wrapping around the largest UFO, making the UFO look like it's in front of hazmat boy. You should also play with how detailed each part of the poster is. Again, the more detailed a bit is, the further forward it looks.