Diferentes finais:

Beat Halo's Campaign on Legendary to see a slightly longer ending movie.

Caverna secreta:

There is in level 5 a secret cave at the botton of the map entrance. It comes out on the side where they are not expecting a side attack from that side and it shall be easier.

Battling flood:

Using a combination of assault, Shotgun, and grenades works best...

Warning: Only use the [sticky] grenades at a far distance, especially for the little ones.

Shotgun - use on big floods, and little floods.  When those little guys, that jump up at you, are all grouped together use the shotgun, it shoots out a lot of bullets at a time (you can see this if you shoot at a smooth wall, or the ground, you can see all the marks it makes).

* Use Assault rifle if the guy is standing still, if he or you are moving too much it moves your aim very badly and you waste bullets and have to re-load more often and then they have a clear shot.  For the grenades use them when you know something is behind a wall.

* For sticky grenades you MUST ALWAYS REMEMBER to use them at a safe distance because if you shoot it on one of those little guys that follow you you will get hurt bad too because of the blast that they brought with them, same with the big guys.

 * Don't use your grenades all at once. They are to important, and sometimes rarest when you need them.

Alternate ending sequence:

Successfully complete the last level of the game in campaign mode under the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both.

Hint: Faster reloading:

To reload most weapons faster, press X and shortly after that press B to melee. Note: For the shot gun this will cause you to stop reloading all together.

Hint: Extra shotgun ammunition:

Take a shotgun that has 12 shots and 0 clips. After you shoot it a few times, you should be able to reload manually. You can do this about ten to twenty times.