Droidican Emperor

From KeenWiki

The Drodican Emperor is a poisonous foe. You'll have to face him on his home turf: The Imperial Palace, which is a strange and unnerving place. The Emperor runs from side to side, throwing poison at you and releasing a robot to help him in his attacks. When you defeat the emperor you get a Plasma Crystal.

There's a switch on the lower left side. Stand on it and press the Select button while the Emperor is in the middle of the screen. Shooting him doesn't work.