Or to do it the non cheating way....
Before you start mission steal cop cars and get the shotty, or go to ammunation and buy it. Anyway you need a lot of shots.
Get a car, you know at the beginning when the three cubans are standing at the entrance using the car as a block?? Run your car right into the "block" car at it's rear wheels. This will cause it to spin and kill the three people. Then continue to drive making that right turn, where three or four more will be coming out of a gap on the right. Run them over. At this time you'll most likely have to hop out of your car because it's going to blow up. So do so. Stand around using the R1 Shooting tecnique and kill the sniper's. Run around the silo conveyer belt sor of thing and kill the two gaurds gaurding the building lance is in. Again use the R1 technique. Run inside the building kill that guy then talk with lance. I think a couple more guys come out, just kill them. If you are facing from where lance is sitting to the outside of the "house" then there is a sentinel sitting on the left outside part of the "house", go get into it. You should have killed every so start driving out, DON'T GO COMPLETELY OUT. Stop right at the entrance/exit to the dump and wait for the game to say the diaz's men are chasing you. You might have to inch forward a bit. Once this is said then you'll be able to see three or four comet's (the porsche looking cars) let them drive down the road until they are quite close to you, then make your escape. You should be able to drive right by them. Then just drive your way to the hospital. If you are a half decent driver then you'll be able to do this without any threat of the three or four comets. And then you have beaten that mission.
The R1 technique I was revering to is that when you have the shotgun or revolver if you hit R1 you focus on the people. Well focus shot 1 one shot unfocus focus again. You automatically focus on another person. If you do this right and effectively you should be able to get through the mission without getting shot alot. Anyway you might want to practive up on some civilians first.
There you have it in as much detail as I could manage, oh and this is how I did it, and it did take me a couple of tries to get it down. Why did you just give up and come ask here?