Only reason obi killed him in the movie was because of shear luck (and lots of plot armor) if you notice, he was only lucky he figured out Grievous's weakness when he tried to damage him in any way possible, and then he also was lucky enough to dodge Grievous's punch (stronger than the death star :O) and then once again, lucky enough to be able to reach that gun fast enough to win. Vader is in Life support, Grievous is, well, Grievous. FIGHT!