If both lightsabers are behind his back, you best start high blocking (Left shoulder + up arrow) as he will spin the lightsabers frequently and will break your block. Hit him now. When Grievous jumps in the air, high block, and he will back handspring backwards. Don't get near him. If the lightsabers are about shoulder height, he will hit 3 times, high block them, then quickly low block, hit him. When he glows and lightsabers high, block and get ready, this down slash WILL in fact kill you. I will warn you now, there is a time after you get the message that he will attack high, attack low, and repeat about 4 times, just keep high and low blocking until he stops, don't attempt to hit him. There is one last final attack, one slash, breaks your block, block again quicky before he hits.