Level 20: Oberon Platinum Mine
Enemies: Class 1 Drone, Class 2 Drone, Medium Hulk, Super Hulk, Cloaked Hulk, Medium Lifter, Driller, Platform (Missile), Gopher
This level features a slightly different mechanic. Each key is held by the Gopher Robot of the same colour. Kill the Gopher and pick up the key. Other than that this level is fairly small.
The Hostages are in a cell in the red area. If you rescue them the nearby Robot Generators will activate. It’s easiest to destroy the reactor and then rescue the hostages on your way out.
Reactor: Super Hulks protect the reactor. You may be able to kill one of them by shooting through the small grate that has a view to the reactor room. Once the hulks are destroyed the reactor should be easy pickings. Make sure you rescue the Hostages before making your way to the exit.
Exit: Out through the Red Door, along the passage, and the exit is the first passage on the right.