DEAD RISING 2: CASE WEST

You only need to complete the Final Boss together to get this achievement. My partner dropped out multiple times, and we still got it both when we defeated him. You can also join just before he gets defeated, you will still both get the achievement. Thanks to Batman5273 and MarioToo for confirming this.

You don't have to do this in a single playthrough. Just reload a save from before beat the game and invite a friend!

Use a save from before you beat the game. One that lets you hunt for supplies just before the boss fight. I used a save personally before I used the C4 from the guards' tower to blow up that one room. Now be sure you have a friend waiting for an invite over Xbox Live to help you while you hunt for some supplies.

You'll need:
- 1 Assault Rifle
- 1 Impact Hammer
- 3 Orange Juice jugs

These are all fairly easy to find especially the OJ. Now, send a game invite to your friend. When they join the game, follow the marker to the door of the final boss fight. The final boss looks like a giant Middle-Eastern man with dual Impact Hammers for boxing gloves. No super mutated zombies here.

After a cutscene involving a woman from Frank's past, the boss fight begins. The boss has the 3 lifebars. Use the Assault Rifle on the boss from afar. When you get the opportunity to get a close hit use the Impact Hammer on him. Run away often and generally keep some distance. Your partner does not need a single weapon. He's there to distract the boss from attacking you. If he needs health, there are plenty of Heavy Rations around the room the final boss is in.

After each lifebar is depleted, the boss will jump off and let a few enemies try to kill you. The Security Guards can be killed with 1 hit from the Impact Hammer. The Zombie Wranglers in white suits with cattle prods can be killed with 2 Impact Hammer blasts. The other guys with their own Impact Hammers have lifebars, so you can tell how many hits you need to finish them. Use the Impact Hammer on them as well and steal the one they drop upon death for a back up or share it with your friend. Steal the Security Guards' Assault Rifles for more ammo.

After you deplete the first 2 lifebars, he will jump up on machinery and you can attack him with the Impact Hammer when he's back on the ground or shoot him with the Assault Rifle when he's up high.

In short, the boss is simple to defeat. It's just a matter of rinse and repeat tactics. It takes 5 minutes to defeat him if that. The achievement doesn't pop until the 3-4 minute final cutscene ends before the credits, so just hang in there for the achievement. Be sure to stay tuned as the end credits feature dancing zombies... what the hell, Capcom?

The cool thing about it cheevo, is that you can join in the begining of the boss fight and defeat him and you will still get this achievement.
Honestly this was my first achievement on this game because I joinned in the middle of my cusins fight, we beat him and puff there it is:)

As mentioned by others, you only need to complete the final boss fight to get this achievement. For me, I went in with:

4 x Orange Juice
1 x Laser Gun
2 x Laser Sword
1 x Shotgun
2 x Assault Rifle

When he's on the ground, run in and swipe away with the Laser Sword, jumping away after every 2 hits to avoid his attacks.

When he jumps up to the second floor, then use the shotgun to quickly take out the Handlers/Mercenaries that come in the room, then use your Assault Rifle to shoot the boss while he's still standing on the second floor.

Just repeat this approach, adding in shots from the LaserGun when he's mid-range or standing on higher ground. He'll soon go down.