So, all you gotta do is in the very first mission with Jackie to rescue the girl from the apartment and have her taken away by the ambulance, just clear out all the enemies and kill the boss. He'll drop a heavy machine gun that shoots an AoE attack with 300 rounds. If you save / load you can refresh the ammo in this weapon. This is what you'll use to powerlevel. Make sure Jackie is at the blue door before you go in and find the chick in the bathtub, then go out on the balcony with the machine gun. Across the way is a food vendor. Shoot him with this machine gun ONCE. If you use any other gun, the game is coded for the hit not to register. If you use the machine gun, you can put him in critical condition, which summons the cops. Once the cops are summoned, mow them down with your 300 ammo AoE super high damage machine gun. Afterwards, let combat end, loot the cops, then run the pathway shown in the video to de-render the food vendor, save in the spot so when you load the vendors death doesn't load into memory, and rinse / repeat for up to rare crafting components and boatloads of exp.
You don't have to use the machine gun to kill the cops, but you do have to use it to tag the food vendor. Also the machine guns ammo refreshes back to 300 every time you save / load.
Have fun gamers.
