Def Jam is like fight for New York, but Liberty City is a best place to fight and gang wars. It's a free-roaming mission.
Bring a player with you to fight with.

I use GTA United: http://www.gtaunited.net/downloads.p6.html
(I have to go back to GTA United due to AIs drives the stupidly in GTA: Underground)

Download Player Mode: http://www.mediafire.com/download/qeldxo7t16a3z8w/Two+Players+with+Models+Pack.rar

Cheats:
p1: Spawn your favourite Girlfriend as seen as in picture
p2: Spawn 3 different actors, friendly, foe, and rival
p3: Spawn Player 3 (Crash)
p4: Spawn Player 4 (Crash)
p5: Spawn a girl
p6: Spawn a love one
p7: Spawn 40+ different actors
p8: Spawn Girlfriends
p9: Spawn Strippers
p0: Spawn Bums