I love dead rising there is a glitch to get around the time limit in the normal 72 hour mode that makes the game a lot easier. also makes it easier to get the 52,396 zombies.
Free Mode Glitch
This glitch allows you have a full free 72 hours game time to complete achievements, without having to do the main missions. You can easily unlock the Zombie Genocide Achievement, by driving over zombies in the Maintenance Tunnels, without having to worry about running to do missions.
Start a new game as normal until you reach the warehouse. Either walk forward, if you have taken the lift, or drop down from the shelves if you have entered through the top door. Walk towards the exit that leads to the mall and STOP. Do not enter the hallway. Just stay there and wait until 7pm game time.
After 7pm you will get the Queen Insect cut-scene, which isn’t meant to happen until near the end of the game. After the cut-scene, you can enter the hallway. The cut-scene which normally happens when you walk through the hallway, where Jessie asks you to find Brad, will not be activated. Since the first mission is not activated, none of the other main missions will be activated. You are now free to enter the mall and wreck havoc to unlock achievements.
taken from
http://easyxbox360achievements.com/