Even if you've collected or missed the other skill shots the left crossover shot stays list for a skill shot combo sequence at the beginning of the ball. Shoot it then the upper loop or side ramp, and repeat for up to 6 shots.
Left ramp is very backhandable.
Soft plunge into the drop targets to get one of the van modifications to boost your next mode, or go for bigger points with a plunge directly to the left outlane.
You can start Area 51 multiball while one of the Overlord multiball modes is running, but not the other way around.