Cheat mode:
Click and hold the Left Analog-stick and press Black, White, Y at the main menu. A "Cheats" option will now appear at the main menu.

Mental mode:
Successfully complete the game in Serious mode, then start a new game. Mental will be fading in and out under Serious. In Mental mode, all the enemies will appear and disappear.

Hint: Defeating the final Boss:
After going into the great pyramid, find the obelisk room. It will then rise. Keep walking around until you see an FMV sequence. You will then go to the place where the Boss climbs up the wall. Beware -- this Boss has the most powerful spells and weapons. After he/it shoots, run to the jump ramps. Jump/boost through the power rings clockwise, and when you are next to the jump ramps, preferably by the backpacks, start unloading with the Lasergun. That weapon will drop him down to a quarter of his health, but only after 800-1200 pulses. Note: He can regenerate health.

When at the Great Pyramid and the door has not opened, pull out your canon and start shooting the Boss in the head. Take out your minigun frequently to take out Kleers, Biomechs, Werebulls, etc. Then, when he is down to red, go inside the pyramid and look up. There are four triangle objects that light up and make strange noises once you jump through the ring things that have lights in them. To activate the teleporter you must use the super jumps (all four) and you will automatically jump through the ring things. However, you must make sure that the Boss is on the lighted up center. Once you have jumped through all four, look up and you should see four blinking triangles. This will take some time, so make sure you have him in the lighted up circle in the middle of the pyramid. If he is still in red when the teleporter is on, and it hits him, he will die.

Hint: Binary counter:
In the Great Pyramid stage, just before you enter the pyramid, you may have noticed a panel above the entry with alternating I's and O's. This is a timer counting down in binary. The timer starts at 1,023 and goes down from there.