Turns out this actually happened! In 1997, a bunch of whale watchers in a boat off the coast of the Farallon Islands (about 30 miles from San Francisco) witnessed an orca attacking a great white shark. Most of the action occurred underwater, but scientists have pieced together the most likely sequence of events.
First, the orca deliberately changed its path to reach the shark and rammed into it at full speed. The shark, which had not been aware of any danger, was stunned and confused. Taking advantage of its confusion, the orca grasped the shark on the back of the head and turned it upside-down. The shark panicked and its brain released a flood of serotonin, a neurotransmitter, that put the shark into a trance-like state. The orca proceeded to feast on the shark's nutritious and delicious liver.
So next time you bump into a great white trying to eat you, all you have to do is flip it over! Easy, right?
For more info, read here: http://www.discoverwildlife.com/animals/great-white-shark-versus-orca