I've seen that a lot of people got the glitch in the Level "Haze" that the Dino freezes and one of the two guys as well. There are a lot of solutions for this glitch and I've tried a few, but for me worked only this one:
Start the Level again, but ONLY with the Pistol cheat: KKtigun
then kill the 2 dinos and go up the platform, where your frind gives you the machine-gun. If he won't follow you, burn the plants down, which are down there, where the second guy is. If he is still not coming down: look at him with A and press then again A to make him do somethink. (if needed, go down again to the second guy, which is waiting where you've come from, to get closer to the guy up there)
UPDATE: (if you still stuck, check the following link)
Thanks to BeatboxingKing! He gave us an additional link to our friends over at X360A!
Thanks to PointPlaceWI! He did the work there:http://www.xboxachievements.com/forum/showthread.php?t=27739