At the "Press Start" screen, press Up, Down(2), Right(3), Left(4), then hold C-Up and press Start to unlock all the Bosses as well as the engineer and a few army characters in multi-player mode. Most of the Bosses have odd guns and may not be able to back up against walls or roll. Alternatively, successfully complete the game on the easy and normal difficulty settings.
Infinite health
At the "Press Start" screen, press Down, Up(2), Down(3), C-Left, C-Up, then hold C-Up and press Start.
Lock on further with the pistol
To lock on further with the pistol, you have to use the machine gun, lock on and switch back to the pistol and it will stay locked on. But if you reload after you locked on, you have to do it all over again.
Max power, sudden death, trial modes
Successfully complete the game on the hard difficulty setting to unlock max power, sudden death and trial modes.
Skip Four Lasers on Roof
If you've been to the level where you have to duck under a bunch of lasers, you know how frustrating it is. Well, here's a way to skip four lasers. First, press the switch against the wall that deactivates the laser lining the wall. Then, place a C4 right at the switch. Crawl under the first two lasers. A guy should come around the corner from behind you to deactivate the switch which is against the wall. Detonate the C4 so you don't have to go back through two lasers, then press the switch, then go back through those two lasers again.
The Endings
Depending on how much time you take, you'll get 3 different conclusions, but it isn't just a matter of reaching the last boss in a certain time. The Gulf satellite takes roughly 2 or 3 hours to charge and fire, and there are two points in the game where if you go over that time, the satellite will fire. So, If you take over 3 hours to get to the Factory Area (stage 23), then you'll trigger the 1st scene. After that, if you take 2 or 3 more hours to get to the Control Center stage (stage 27), you'll trigger the second scene. To get the Good Ending, in which you fight Dan and Cecile, you can't trigger any of those scenes. To get the Normal Ending, in which you only fight Dan, you can only trigger one scene (doesn't matter which). To trigger the Bad Ending, in which you only fight Cecile, you have to trigger both scenes.
Trial Mode (Play any Level)
At the Main Menu, press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left, C-Down + Start.
All multi-player characters ...