I'm not to sure if anyone has mentioned this before and sorry if they have, this is basically for people that don't play the story. I'm not sure if it works for the 360 achievements though. And i also think you need to have gta4 installed to do this too. Requires TBoGT DLC
So all you need to do is go to the cage fighter place and bet all your money on a fighter but make sure you save before hand and also if you win so you don't lose anything.