Dooku was at first taken aback by so bizarre an underling. But he and Darth Sidious saw potential. Dooku began training Grievous in the ways of the lightsaber. He was dismayed that the subtleties and finesse of lightsaber combat had been discarded in favor of brutal, multi-weapon attacks, but so go the changes of war. At his hidden base aboard the Trenchant space station, Dooku pit his minions Asajj Ventress and Durge up against Grievous to determine who would be the commander of the Separatist droid army. Grievous emerged victorious.
Yes, but it could have been because of his surroundings, his weapons, etc. Just because he did it, doesn't mean he can do it again every time. Same with Luke and the Death Star.
Now, Asajj Ventress herself is a match for Anakin or Obi-Wan, the latter of which beat the shit out of Grievous. Durge is a monstrosity. The idea of Anakin and Obi-Wan beating the pair is a stretch. Saying Grievous beat them makes me question the way he beat them.
And since no one has exact details, why should I be convinced by that?
And yet the actual fight only took place in Star Wars Tales. Unless you want to believe Chewbacca is the Sasquatch, the series may be disregarded.
Either way grievous wins for sure. His armor is laser proof. The only reason Obi-Wan shot him was because he just happened to have pulled back his armor, exposing his organs. If Jango tried to get that close he'd end up like he did in AOTC.