Get a sniper rifle or two from the weapons shop in the palisades mall. When the battle starts, run around to the back of his platform, on the left side from where you come in. look for discolored parts of the floor and get him to throw beacons for the plane to attack there. Find a spot to make a hole directly above some scaffolding on the floor below, so you can stand on it and still see Sullivan. From here, just snipe him until he dies.

The easiest way I've done it is this

First get a LMG (its the gun with about 200 shots, you can get from one of the casino robbery areas)

Make sure you have food. ( I recomend Steak or 2 blended steak. They provide alot of health when you need it)

Now when you climb up roll or run around and wait for him to roll. Then give him a kick ( RT + square ) and he'll be bent over.

Unload on him. and Repeat. It may take a few longer then what they posted but it will be alot easier in the long run.