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The Mi-24A Chopper mission in the Caratara De La Muerte is a difficult mission. Chopper comes with 20 troops (5 waves of 4) and all troops deploy directly from the helicopter. This lends some leeway in the way of creative problem-solving.
For a straight fight, use armor or jungle fatigues, a machine gun (armor piercing if available), and as many rockets as you can carry. When the helicopter deploys more troops, send a rocket into the hatch where the soldiers are jumping out. This will reliably kill 3 or four troops. once the troops are dead, damage the canopy until the captain pops out. A rocket will kill him in a single blow, helmet or otherwise. Use ariel mines to stop the chopper in the air, or fire your machine gun at the fuel barrels under the wings to do serious damage and demobilize the unit.
For S rank, stealth is key. Bring Night Vision Goggles, Fulton balloons, Fulton sleep mines, smoke grenades, a cardboard box, some holdup weapon, and a silenced tranquilizer gun. Throw smoke grenades at the troops when they deploy, and abduct all the troops. If you are not spotted, the canopy will open. Tranq the captain. The hardest part is staying out of the helicopter's line of sight. there is no way to blind the chopper, so just be careful, and don't be too sad when you need to reset. Take all the time you need.
Next: Head for the Lab