Level skip
Pause the game and press Up, Down, Left, Right(2), Left.

Invincibility
Pause the game and press Left(2), Right(2), Down, Up.

Maximum ammunition
Pause the game and press Left, Right, Left, Right(2), Left.

Refill health
Pause the game and press Down(3), Left, Up, Right.

Unlock SegaNet levels
At the main menu, select the "Network" option, then choose the "Account" selection. This should switch to "Account 2". Use the following configuration:

Login name: admin@sega.net
Password: tenages (Seganet spelled backwards)
Area code: Type in your local three digit area code (Ex. 123)
Phone number: Leave blank (do not type in anything)
Primary DNS or DNS1 = 0.0.0.0
Secondary DNS or DNS2 = 0.0.0.0

Click on "Connect" and the game will ask if you want to save changes. Click on "Yes". You should now have access to all levels in deathmatch mode (if you already have completed the game or used the "Level skip" code). Go to practice mode to find two new levels that were previously locked. Note: Make sure you do this on "Account 2" or your online configuration will be overwritten -- and you will then have to set everything up again