3 Dual Screen
Game Boy Advance
The bonus levels are alot like the ones found in Gradius III on the Super Famicom, they have some assorted extras, like 1-ups, point pickups and the best new one, auto-fire.
Also, in addition to the auto-fire power-up, there is a scroll stop item, much like the one found in Gradius 2 MSX.
Towards the end of the stage, you need an odd number of points in the hundreds column to access this bonus stage (00100, 00300, etc)
Along the bottom path, right before the boss, you need nothing highlighted in your power bar to access this bonus stage.
You must fly in between the lava and the inverted volcano.
Thanks to King Moai