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The Facts - Catching Sullivan
Sullivan has a bit of a headstart, so our first objective is to just catch him.
Advance and leave the safehouse. Note that this isn't really that hectic or timed. If you want to swing by Juggz and get some painkillers, then feel free to go ahead and do it. Leave the Royal Flash Plaza for the main strip to get the message that Sullivan is waiting on top of the Yucatan Casino and is signalling a plane for a quick extraction.
Note that the preparation for this is easy. Get as many painkillers as you can carry. Don't worry about melee fighting. You might want to bring one melee weapon for clearing out zombies, but that's it. Bring a few pairs of knife gloves, since they are about the only weapon that will work well against Sullivan. If you want to use guns, then gather up some guns at the gun store in Palisades and grab the LMG from the Yucatan.
When you're fairly well equipped, go through the Yucatan and go through to the marked elevators. I strongly suggest that you save your game in the Yucatan bathroom. It's a real pain to have to start over at the Safehouse and do your prep again. These elevators will take you up to the level below the rooftop. Cut through the kitchen to dodge most of the zombies. You can also get on top of the bar to reach a safe area and drink some of the vodka laid out around the bar. There's also plenty of food in the kitchen if you still have room.
When you're ready, fight your way up the stairs to get to the rooftop and find Sullivan.
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The Facts - Sullivan's Tactics
First, Sullivan has a revolver that packs a fair punch. He'll take pop shots at Chuck as long as he has the ammo and he has a shot. The shot also staggers chuck, so it's a bit annoying.
Second, Sullivan will throw flares around. These flares will act as the beacon for his air support. After a short period, the gunship above will hammer the location and blow a hole in the floor to the construction zone below. Falling through isn't good, but it isn't necessarily a true disaster. Note that it is possible to pick up a flare and throw it onto Sullivan's platform. This should call in an attack run right on top of him and do a fairly large amount of damage.
Sullivan's worst attack is his counterattack though. If you try to attack Sullivan with a melee weapon, he will attack you and disarm you. This will possibly end with Chuck being thrown off of the raised platform through a hole. If you survive, just take a break in the kitchen and heal.
- slide 3 of 3
The Facts - Killing Sullivan
If you're using guns, then good luck. Bring whatever you can and just trade fire with Sullivan. Go ahead and see if you can hit him with his own flares to make the long fight a little easier. One important thing to note is that you can leave at any time and Sullivan will not heal. There is nothing stopping you from going back down and grabbing a new LMG if you want.
If you want to use melee attacks, then you need to climb up the stacks of platforms and get onto the raised portion with Sullivan. There are a few options. Knife gloves are immune to Sullivan's disarming attack, so they're a good way to pack a heavy punch. Get close to Sullivan to bait him into an attack and then counter with your own. Rinse and repeat. If you need to heal, drop down a level on the platform and drink a painkiller.
If you can't make this work either, then there is one guaranteed and cheap way to kill Sullivan. Use the double-leg dropkick or jump kick him to death. The jump kick is a bit of a pain to hit, but if you're leaping around he won't be able to hit you. It's a death by a thousand cuts, but you should have plenty of time as long as you don't get bored. The dropkick does a lot more damage and seems to connect a lot more. Using the drop kick will cause Sullivan to fall to his knees in pain. If you're really quick, you can even get in a few hits with your fists or the knife gloves. Regardless, you'll be able to stand back up in peace and just repeat the process until his health falls to 0.
Regardless of how you do it, enjoy the final cutscene. If you chose to give TK Zombrex, then you have one more section to do. It's time for true overtime mode.