R2 should be able to hover to the astromech locks- operating these will lock the two doors the troopers come from, ending the constant attack. vader will then start throwing things at you using the Force- once they get past halfway to you use the Force yourself to throw them back at him. After he's hit a couple of time Vader will come to you for a fight. After a couple of hits he'll go back and the process is repeated. Once vader is out of hearts you still have to get to the point where you Force back the things he throws, only instead of hitting him it smashes a window and he jumps through. Build a ramp out of the broken glass using the Force and go after him.