During Last Boss there is a invisible timer to finish the game. During this time you must destroy all little enemies up and down in the screen (leeching), and attack the Boss (count up all shots against the last Boss).
You must know the exact number of shots to destroy the Boss.
You must Know the exact time that you have before the auto-distruction of the Boss.
When the Boss have 1 damage/shot remaining, and time count is near to finish, you must shot to destroy the Boss and at the same time you must explode. This is Double K.O. system.
If you are perfect, 50k total for each life.
Is difficult? Yeah, but is the only way to obtain a very high score.