Grevious was able to take down the Jedi he did through a combination of multiple factors. 1) Underhanded tactics. He had no issue with things like having a MagnaGuard stab a foe in the back or a sniper shoot them down during a fight. 2) Superior stamina/No fatigue. He could theoretically fight for as long as it took, while his organic opponents would wear down and run out of energy. 3) Sheer, raw power.