My son is having the same problem so we will wait for answer as well

I just figured it out last night at the bottom of the platform grievous is on a box falls out of the door on it and you build it or use the force on it and then jump up where grievous is and attack him then a little while later in the level you have to fight grievous again but you can't hurt him when he is attacking so let him chase you a little bit and he will start coughing that is when you attack him repeat this until his life bar is gone.