How to unlock and complete
To unlock it, you must complete the other 8 of the Megalodon's missions. For completing it, eat banks of creatures to get a high multiplier, and then eat some creatures to score points and undergo a gold rush. Repeat this until you complete the mission. If you must/want to, revive yourself using gems.
Since 5 million points are quite a lot and the Megalodon has the tendency to be very ferocious for a while but then die quite suddenly, it is a good idea to start your mission in the Arctic, where Cold Rushes give you even more points than Gold Rushes.