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Then just walk further into the complex and you'll see a sniper rifle. Pick it up and snipe as many Cartel as you possibly can, then shoot up the Flatbeds. You should then have a clear run at the steps to get to Maria. Don't go right up the stairs though, walk up slowly and pick off the guards with the sniper rifle. If you do it right, the guards won't even notice you. Just remember to keep out of the Helicopters line of fire.
Then walk to the edge of the platform and pick up the rocket launcher, one well aimed rocket later and the helicopter will be toast. That's how you do it without using any cheats. This is probably my favourite mission in the entire game.
Don't go too close to the flatbeds, I think the maximum distance is near those red and yellow containers on the left. Use the sniper rifle to kill any of the guys nearby and a rocket launcher for the flatbeds. Destroy them all. By this stage Catalina will be in the chopper and they'll drop a few molotovs on you, therefore ensure you have full health and armour when you see the chopper coming at you. Down it with a rocket launcher, only takes one or two hits. Next, go to the stairs (ensuring all the Colombians are dead) and go up the stairs. There are three Colombians guarding Maria. Walk halfway up so you can use the sniper rifle but so you have good cover, and pick them off. The hardest is a guy to the far left behind concrete structures, you may need to go closer to get a good shot off at him. Finally go over to Maria. Mission passed.
Hope this helps.
"Now then, now then, now then"