Pause the game, press the A button 8 times and then unpause. This can only be done if you're alive.
100.000 bonus points
In the Graveyard stage, survive for 1 minute and 30 seconds without triggering the boss appearance (don't kill too many enemies). The stage will change to night-time and you'll get 100.000 points.
El Doba's True Form
In any stage, defeat El Doba (the boss) before he fires 10 shots. In the next stage, don't lose any lives. Instead of his normal form, El Doba will appear as a schoolgirl and will shoot faster.
You can do it as many times as you wish, so El Doba appears on his true form on all subsequent stages.
Hidden Character and 10.000 bonus points
On the beginning of any stage, the number of hundred's value in the score has to be either 0,1,4 or 5. Then, the 1st shot you fire on the stage has to hit the 3rd enemy that appears (you can't miss).
If done correctly, a character - that changes depending on stage - will appear at the upper part of the screen and net you 10.000 points.
A easy way to test this is at the beginning of the first stage, since the score will always be 0.
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