Grievous blindsided five terrified and exhausted Jedi who'd just been completely and unexpectedly routed. He didn't take them in a straight fight like he did Obi-Wan, he fought smart. That's how Dooku told him he needed to work if he were to stand any chance against the upper-tier of the Order, even going so far as to tell him to retreat if any one of those elements (fear, intimidation, surprise) were absent.