I recommend saving before this mission so that you have a save and full health at the start. I also recommend grabbing the body armor from behind the biker bar before you start. First you will need a fast bike. I recommend a PCJ 600 but if you don't have one and don't feel like looking for one the angels parked outside the bar will do.
Drive the bike to the marker at a good rate of speed and you use the stairs at the marker to jump to the roof of the ammu-nation across the street. Once you are on the roof get off the bike and move to the yellow marker which is Tommy's bike. Shoot any gang enemies that get in your way but don't waste time trying to clear the area they will just keep spawning.
Once you get the bike drive left and clockwise around the area. Once you make two rights there is another staircase you will see that you use to jump out of the enclosed area. Once out just drive back to the bar. The gang will chase you in vans but shouldn't be a problem. If they do manage to get you off the bike either get back on quickly or shoot them and go. The bike is extremely hard to drive if the back tire gets popped.