oddeye: get right over them in a helicopter, start the helicopter in a violent and directly vertical freefall and jump out as late as possible. usually works. you can cover the helicopter in contact explosives first for bonus points.
I did pretty much this exact thing in Mercenaries as well, with helicopter winches and random jeeps. They left neat fire trails in the air due to impact explosions + fifty-foot bounces.
Just Cause 2 makes a grappling hook more fun than I thought possible, and that's saying something.