Dead Rising 2

you MUST complete all main missions, No Survivors are needed to be rescued.

Major Spoilers

For the fight with Sullivan, I recommend using firearms to avoid Sullivan's melee prowess, disarming abilities, guns, and the horde of zombies nearby. I hid behind a big, square hedge. I had no problems with zombies or the plane's missiles, and Sullivan politely stayed on his scaffold (he'll still shoot you).

There's a sniper rifle in a hidden area above Players on the second floor of the Royal Flush Plaza. You can jump up using the red phone booth and will have to do a small bit of back and forth platforming. I found that one gun wasn't enough, so try to find another one, including by leaving the Royal Flush Plaza and returning to respawn the sniper rifle.