The thing you gotta consider is, if Obi-Wan didn't get lucky with that Blaster pistol being nearby, Greivous would've killed him, Grievous was easily outmatching Obi-wan for near enough the entire fight, the only reason he actually succeeded in the end was because that blaster ignited Greivous' insides.
By the end of that battle, Obi-Wan was hanging on the edge of a landing pad, clinging on for dear life when Greivous was about to finish him one of his Vanguard's electro-staff, Obi-kins got lucky with that blaster being there. ._.
I reckon at the very least, Greivous would be a damn fine challenge, if you look at his history through the original Clone Wars anime, as well as a significant portion of Star Wars canon, he took on and killed countless fully trained Jedi knights with significant control over the force, he's the reason why the Jedi numbers were dwindling in the first place.