Just use the sniper rifle to pick off all the incoming soldiers on the second wave, then use the mounted gun to kill the heavies quickly. If the heavies are already too close, just get up and personal with them. All it takes is a few shots to the head, then unload the rest of your ammo clip on the pack on their back. They are really easy and don't seem to do a ton of damage, at least on normal. I imagine it would be much more difficult to kill them with a sniper rifle anyway.
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