Unclock The A, B and C Battlegrounds:

Go to one player and choose endurance. If you defeat 10 cars you get the C battleground,

20 you get the B battleground ,and 30 you get the a battleground.

Get Trapper:

For those who do not have a Game Shark, to get Trapper you must defeat the game

with Mime, Darkside and Axel. To get Axel you must go to the playground level

and shoot the middle swing with a missle. you should see the icon of axel pop

out and pick it up to unlock him.

Perform Special Attacks:

Freeze: Press Up, Down, Up.
Shield: Press Right(2), Down(2).
Rear Fire: Press Left, Right, Down, L2. 
Drop Mine: Press Right, Left, Down.
Jump: Press L1 + R1. 

Earn Darkside:

To earn darkside go to the kitchen level. Shoot the cabinet doors on the counter

in the middle of the room until you blow holes in them. Go inside the counter

and you should find an icon of darkside.

GameShark Codes

Infinite Battery Attack Power   
80151e882a30 

Infinite Environment Weapon   
80151ebc0003 

Infinite Health   
801504a004fb 

Infinite Homing Missile   
80151ea40003 

Infinite Lives   
80160f8c0003 

Infinite Napalm   
80151eac0003 

Infinite Power Missile   
80151eb80003 

Infinite Remote Drop Mine   
80151eb40003 

Infinite Ricochet Bomb  
80151eb00003 

Infinite Roman Candle   
80151ea80003 

Infinite Special Attack   
80151e9c0003 

Infinite Turbo   
801504ac03e8 

Inifinite Fire Missile   
80151ea00003 

Unlock Axel   
301d1e3c0001 

Unlock Darkside   
301d1e400001 

Unlock Mime   
301d1e440001 

Unlock Piecemeal   
301d1e480001 

Unlock Trapper   
301d1e4c0001

Thanks to Revolution readers MAC, Tison McCartney, Jay C, Chris and mike!