I'm pretty sure that you need to do those missions to finish the story, so, unfortunately, you can't skip them. If you're having trouble with the mission where you first steal the Buzzard, you can mark the spot where the guys in the boats reach land, park the helicopter there, and wait for them with an RPG. The rest aren't as bad, iirc.
"Damn straight. I don't care what my relation is to that woman, I'd bang her six ways from Sunday. And six ways UNTIL Sunday." - Blue Dragmire