Kit Fisto is a master of Shii-Cho (a form that excelled at fighting multiple weapons). It's pretty obvious that Kit would win. And on that topic, IIRC Grievous killed several of the clones... as a torso.
Now when I said that the assassins wouldn't work well, I noted that the guard droids may (MAY) be able to see them. Not only that, but the assassins aren't going to be that great against Grievous. Remember when he beat five Jedi Masters on Hypori (if that is still canon)? The more desperate he gets, the more dangerous he gets.
And no, an assassins can't just walk up behind him and "whack" him. He would have to complete disassemble Grievous and destroy his organs. Which means the assassins is forced into a direct confrontation because he has to remove that protective armoring. He won't be able to do so in stealth as Grievous would notice his body being tampered with.
Revan has to engage Grievous personally, and he would most likely run from a fight he can't win.
This is a challenge, but I wonder if a projectile weapon could puncture duranium. Not much is said about it on wookiepedia. They might be a to use ion weaponry-but I don't know the effect that would have on Grievous. Or use explosives. The heat might become high enough, or the shock wave might kill him.