The first time I didn't realize I was supposed to jump off that ramp until it was too late. The second time I hit a barrel right before the jump, and I finally got it the third time. The whole first part of that mission is dead easy. The shootouts get really boring and you don't even need to use the cover. Just use the AR-15 and zoom and use auto-aim and then manual aim like 1 foot higher for a headshot and you can kill almost anyone with one bullet, even when they're behind cover.