Unlimited continues:
Press Up(2), Down(2), Left, Right, Left, Right, A + B at the title screen.

Sound test:
Press Up/Right + Start at the title screen.

Level Password
1-1 LPRZ
1-2 MDRR
1-3 (none)
2-1 NMLL
2-2 NWKL
3-1 LGZQ
3-2 GPTW
3-3 (none)
4-1 GNXF
4-2 KHCN
4-3 (none)s
5-1 QGVN
5-2 WBZT
6-1 FFHG
6-2 CKQG
6-3 (none)

Defeat moth Boss:
Pause game play and press B, A, B and A. The moth will fly away when the game is resumed.

Defeating The Joker's bodyguard:
Use the Batarang and kneel, but be sure you jump over his energy blast.

Defeating The Joker:
When you fight the Joker, rather than use a weapon, use your fists and kneel. When he summons lightning, just step away from him. Make sure you also follow him when he moves across the screen.

Unlimited power-ups:
Locate the falling drop claws, then crouch next to where they are landing. Face the claws and repeatedly punch in this direction. Each defeated claw will transform into a power-up. Use the pattern that the claws are falling to collect the heart, weapon bonus, or extra points before returning into position to hit more claws.

Level 2-4: Avoiding damage:
At the end of the second level, if you destroy the two guns and the blue eye on the left side, wait for the blue eye on the right side to shoot and then jump up and duck right next to it. If you get close enough, you will not get hit by the eye or the bullets. Then, jump up and punch it to death.