I haven't seen one here yet, so I thought I'd make one. And seeing as it's quite a hard mission, and a lot of people get stuck on it, I have made a guide to help you all.
Weapons recommended to use for this mission:
AK47, M4, Micro Uzi/Tech 9, and any other weapons of your choice. Those are the main ones though.
Before I start, it's a very good idea that you have a minigun on you, it will become useful later on in the mission. Also, it's best to do the firetruck missions before you attempt this, it also will be extremely helpful as you progress through the mission.
Walk into the red marker outside Sweet's house to trigger the mission.
Okay, run up the stairs, kill the five or so Ballas, and dodge their bullets, crouch and roll, that helps when trying to dodge bullets. When they're all killed, pick up the armor icon near the entrance to another part of the Den.
Now when most people are dead, head to the end of the room and up the stairs. Kill anyone that gets in your way.