This may be a bit obvious but what the heck. Purchase the Whipfist upgrade and I think one of the grab upgrades. Use Whipfist to easily hijack helicopters/tanks. Very useful with tanks if you can't get close, equally useful for Helis since they keep flying up and up.

Play the game until you reach the mission called The Last Man. By then the majority of the city should be infected. Hijack a helicopter and use infected vision and fly into an infected zone. Look for infected watertowers and blow the crap out of them. Alternativley blow up hives or bases for big EP's or just go around and slaughter infected people in hopes of getting the trophy Trail of Corpses.