Hmm. Are you looking for tips on just the robot Heavy head or the the whole botkiller sniper rifle? What kinds of materials do you have/want to use and what kind of budget do you have for the costume?
For the head you could probably use a big styrofoam ball as a base or even better, a foam halloween skull and sculpt over it with something lightweight like paper mache (or with something else if you want to cast the finished thing in another material anyway). Or you could try to scratch build it from cardboard or something... Whatever you make it out of, you could hollow it out and add LEDs to the eyes too. Helped a friend make a servo-skull for a 40k costume and that comes to mind.