Dead Rising 2

Credit to Finalsight618 for the original information in his 360 solution. This is my spin on that solution with a little extra detail.

Edited to include information from comments thanks to Lyco499

Start a new game and when you are free to start the first mission of getting the Zombrex, ignore that. Go to the restroom in the safe house where the save point is. Look to the left and find the clothing locker. Go up to it and press then to the tab that reads Bonus Content. Select at least one piece of the Ninja outfit (makes zombies ignore you) and one piece of the Sports Fan outfit (makes you run faster and you don't get sick drinking alcohol). This combination will make the game comically easy with regards to regular zombies and will make the gambling hassle free.

Now collect the three gambling books in the following locations:

Gambling 1 - Second floor of the Royal Flush Plaza in Ragazines.
Gambling 2 - Second floor of Palisades Mall in Stan's Large Print Books and Magazines
Gambling 3 - Silver Strip in the Shamrock Casino behind the counter

After you collect the three gambling books, go to the Slot Ranch Casino. Go through toward the hallway leading to the food court and you will find a huge video slot machine against the wall with two buttons. Play the one on the right for $1,000 per spin until you reach $7,000,000 so that you have extra for other purchases if necessary.

I had heard about Dead Rising 2 crashing a lot, so I went to the restroom there in the Slot Ranch Casino to save after every $1,000,000 and along the way, went to a handful of the lower money slot machines with the money laying on the ground in front of them. You can play them for 5 straight guaranteed wins ranging from $1,500 to $3,000 each win, which is $7,500 to $15,000 per machine. This can also be used to establish your bankroll prior to playing the big machine. Now do the same on the way back to the big machine. I kept playing, saving, and hitting the smaller games until I had $7,000,000 just to be safe.

Now save, quit, and restart and go get the Zombrex and give it to Katie, then do the mission "One Man's Trash" (located at Moe's Maginations on the Platinum Strip) to unlock the pawn shops. I suggest buying the Knight Boots inside this pawn shop for $2,000,000 then head up to Tinkerbox, the pawn shop on the Silver Strip, across from where the SUV is located to buy the SUV key for $2,000,000 and then into the Royal Flush Plaza to Just In Time Payday Loans, the pawn shop upstairs, for the Sports Car Key for $500,000. It was shortly after this that the achievement popped for me, so it apparently counted the money that I put into slot machines.