Okay, I'm finally up on the mountain, and there's probably a combination of things that got me there. i thought it would be nice to share, if not, stop reading 
1) I got better at shooting
2) I got a lot closer all the time than I actually thought
3) The walkthroughs gave me an idea that apparently has worked. They all - or rather: they BOTH said that you should get on the other gondola as soon as possible to start shooting the planes.
Instead, I didn't. I shot the two soldiers, and after that, waited as long as possible on the gondola that I started on. This gondola will take a lot of damage, but the other one, will not take one single hit. So the longer you wait, the better the chance that you'll make it to the mountain. Don't stay on too long, because either the gondolas will be too far apart to make the jump, or the gondola that you're standing on will take so much damage that that one will drop down with you on it.
