Few of 'em:
1. Try to take as many guys as possible with sniper rifle while standing on the higher ground
2. Throw some molotovs on Colombians' positions. You will know if you hit them if you'll hear screaming
3. A bit of cheating - use car (the best is ambulance, but Cartel Cruiser also will work) to jump across the fence, as close as possible to the blip and run straight to the elevator.