Beat the Campaigner
When facing the Campaigner (the final boss), take out your Particle Accelerator and start charging it. As you charge it the Campaigner should stall and stand still. Now fire it at him and change your weapon (the minigun and auto shotgun work well.)
Even though he has a forcefield he'll start laughing as he blocks your attack. This leaves him open for your second attack. Repeat this process for an easy victory.

Cheat Codes
Enter these codes in the "Enter Cheat" menu. To use them, go to the Cheat Menu and enable the desired codes.
All Weapons - CMGTSMMGGTS
Big Heads - TSHNTTBNCTPRDCRD
Dana's Cheat - DNCHN (shrinks enemies)
Disco Mode - SNFFRR
Flight Mode - LKMBRD (use R and L to move up and down) This is an awesome code to make the stages ten times easier!!
Gallery - THBST
Greg Mode - GRGCHN
Infinite Lives - FRTHSTHTTRLSCK
Pen and Ink Mode - DLKTDR
Purty Colors - LLTHCLRSFTHRNB
Quack Mode - CLLTHTNMTN
Robin's Cheat - RBNSMTH
Show All Enemies - NSTHMNDNT
Show the Credits - FDTHMGS
Spirit Mode - THSSLKSCL
Unlimited Ammo - BLLTSRRFRND
Grenade Launcher in the Ruins
When you get to the temple with the save point on top, save then look around. Look to your side and you'll see a ten-point Life Force. Walk toward it and you should see a sparkling path. Follow it and you'll find the Grenade Launcher.

Skip Across the Water
Basically, this little trick will allow you to jump on the surface of the water repeatedly to the other side. To do this, fall off the ledge toward the surface of the water, but tap the jump button as you hit the water. You should jump off the water! If you get the timing just right, you can jump right along the surface of the water!

Ultimate Code
For level select, all weapons and much more, enter the code NTHGTHDGDCRTDTRK at the "Enter Cheat" menu.