Dead Rising 2: Off the Record

Those achievements can be done in one playthrough (IF YOU ENTER SANDBOX MODE FIRST AND COMPLETE ALL CHALLENGES BEFORE DOING CAMPAIGN MODE YOU WILL MISS "PUREWALL MEMORIAL CUP" ACHIEVEMENT).

The first thing you will need is rescuing Denyce (The first case of Dead Rising 2: Off The Record (Frank needs to find Zombrex)), Just go to Medical store (Roy's mart) by following marker on your screen. When you will get there, you will ecounter 3 looters which you need to kill before rescuing Denyce. After you killed them just talk to Denyce and whe will follow you, afterwards continue playing with her for the rest of the game. (by not going with her to safe house, until (CASE 7-1) since it will ruin the achievement for you).

On your way back to safe house you can pick up "Orange Juice", which is necessary for second achievement for being "Vegan" until the military appears (CASE 7-1)

In order for her to stay out of the Safe House you can use RMB (Right Mouse Button) and Q to aim and mark the place where she should stay (Just make sure she doesn't have green marker above her head with Door or otherwise she will warp to safe house and you won't get this achievement). Also make sure that on CASE 5-2 you won't go on rooftop with Denyce, since the ending of that case brings you to safe house automatically. Just make sure she has a lot of HP to survive this long, by giving her food you won't ruin the 2nd achievement for yourself.

Or the second method for Alpha Vs. Omega achievement is to find someone with a savefile where they are with Denyce on CASE 7-1 and join them and play in co-op on this savefile.