He barely survived against THE Soresu Master and The Chosen One. That's pretty good if you ask me.
How exactly is Revan going to get assassins to Grievous. Revan doesn't possess stealth. Not to mention that guard droids may be able to see stealthed assassins.
Then there's the time that Greivous barely survived an surprise attack (in his own home..... easily infiltrated) from Kit Fisto, his former Padawan, and a squad of clone troopers. Not only that, but I think they were trying to take Greivous prisoner. Revan won't be trying to take him prisoner.
So that there prooves that you don't even need steal to beat Greivous. Greivous will be confronted by Revan, possibly Malak, and a host of Assassins who can appear and disappear at will. Greivous will go down very easily in such a situation.