Meet up with your business associate at the bar to start the mission. After getting to know Mr. Cheng a little bit, get into his car and head to the kitchen. Along the way, you call Chef and he says the Aztecas are attacking the place. A short scene will play out and then you'll be shooting out a window, looking out into the lot below.
If you have auto lock-on enabled, keep pressing the aim button to get a bead, get a few shots off, then go back into cover. After the first wave is finished, follow Chef through the one door and go back outside to find some more. Do the same thing you did before with the cover until you clear this wave.
There will be one more wave up top, so go to the back of the building and use the concrete wall as cover. After a few shots, Chef will toss you a grenade launcher, so use this to get rid of some of the vehicles that will start pouring in. Once it's clear, follow Chef downstairs to the building's entrance, where you will kill a few more Aztecas. They will run off, so check on your friends to finish the mission.