In Season 4, Episode 10 of Star Wars: The Clone Wars, General Krell is shown killing dozens upon dozens of clones while under heavy fire. However, during the execution of Order 66, it takes only a handful of troopers to kill Master Ki Adi Mundi (which you can watch and cry over here). How is Krell able to resist an entire company (or more) of troopers simultaneously firing on him?
The biggest one: The element of surprise was the key advantage that the clone troopers had in executing Order 66. The Jedi with whom they were fighting did not expect an attack from their own troops, and weren’t watching them, either with their regular senses or their Force senses. Then, too, Order 66 was made possible through biological chips implanted in the troopers, which made them act mechanically and without malice. This could also have limited the ability of the Jedi to sense them. Jedi have senses for danger and limited precognition, but they can’t sense every incoming threat, particularly under the circumstances detailed in the second point. Some will slip through.
In Pong Krell’s case, though, he knew that his troopers were against him. His attention was focused on them, and they initially even explained they were taking him in. It’s really the exact opposite of the situation the Jedi found themselves in during Order 66, and one that heavily favors the Jedi.
Then, too, the power of the dark side grew ever greater with time, diminishing the ability of the Jedi to sense things through the Force. At the time of Krell’s betrayal, the dark side was likely not as ascendant as it was during Order 66. Besides, he had clearly fallen to the dark side: it would more likely have aided him than hindered.
The Jedi who were killed during Order 66 were often in combat with enemy forces, thus distracting them and focusing their attention on things other than their own side. They expected their clone troopers to have their backs, right up until the moment they didn’t. Pong Krell only had to contend with his own forces, as I recall.
There are substantial differences in Jedi combat ability, stemming both from training and intrinsic strength in the Force. For example, Anakin Skywalker, with the highest known midi-chlorian count in the prequel trilogy, consistently showed the ability to take on more droids than other Jedi. Yoda and a handful of clone troopers took on hundreds of Separatist droids, whereas Luminara was taken prisoner by a number of Geonosian zombie drones. Kanan and Ezra were hard-pressed to deal with two Inquisitors, whereas Ahsoka defeated them without effort.
Most of the previous examples likely have to do with intrinsic Force strength, but combat training is not irrelevant, either. There were seven forms of lightsaber combat taught in the Jedi, some of which focused on blocking blaster fire, some of which were most useful against opponents wielding the dark side of the Force, and so on. Different Jedi would specialize in different forms, and this could dramatically affect their effectiveness against blaster-wielding enemies, such as clone troopers.
Thus General Krell might simply have been stronger than the average, or highly trained in, say, Form IV, or combat overall. He did seem to have a stronger predilection for violence and combat than the average Jedi.