Easy

(preferred) Shoot up the middle which will raise ball to upper play field. Toggle between the lower and upper shots (whichever is lit). This is the safest way to play.

Medium

Qualify locks by hitting targets on either side of middle shot, then shooting middle shot.

Hard

General notes

The game has a lot of kill you shots, so be careful on a fast/aggro machine.

Rafts can be worth a reasonable amount after you have collected a bunch. I like to go for the raft if its on a safe shot.