Nemesis

Level 2
Destroy the volcano towards the top after the ceiling drops and rises. Fly through the hole where the volcano was located.
Level 4
Destroy the second Re-Bone blocking the narrow passage near the end of the level. Fly to the bottom of the passage to reach the wall behind your ship.

Pause game play and press Up, Select, Down, Select, Left, Select, Right, Select(3). Collecting power-ups will have no effect on your ship.

Start the game, then pause it by pressing Start. Now press Up, Up, Down, Down, Left, Right, Left, Right, B, A and Start.