Self explanatory. Just complete "L.A. Meltdown" episode on any difficulty to achieve. You may also use cheat codes to make things easier if you wish.
Useful cheats if needed. Just type the code on your keyboard while in game to activate.
DNKROZ - God Mode + Unlimited Jetpack
DNSTUFF - All Weapons, Inventory & Keys
DNINVENTORY - All inventory items
DNWEAPONS - All weapons
DNKEYS - All keys