High Dive - For the headshots/accuracy, use the AA-12 and aim for the head. Unfortunately, it's not possible to destroy the 5 helicopters and have accuracy because accuracy is based on hitting people and is reduced by hitting anything else so you will have to make at least 2 playthroughs. For "Freefall Time," I believe this is similar to the Adrenaline Junkie achievement so pull the cord at the last second
It is absolutely possible to get the accuracy and helicopters. Just use the RPG and so long as you hit the helicopter it will keep your accuracy.