Secret Cave:
There is in level 5 a secret cave at the button 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.

Get New Ending Sequence:
Finish the game using legendary difficulty setting in Campaign Mode

Carry 3 Weapons:
OK to carry 3 weapons u have to beat the game (i beat it on easy) then when your done you can carry 3 weapons

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.

Shot gun- use on big floods, and little floods.
When those little guys, that jump up at you, are all grouped together use the shot gun, 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).
Also close to medium range range to the big guys.

Use Assault rifle if the guy is standing still, if he or you are moving too much it screws 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, or when you see a lot of flood coming at you from a safe distance.
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; I have learned this because the the Master Chief doesn't run to fast so he cant out run a lot of the flood.

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

Battling the flood 2:
When you fight the flood it is best to use a combo of the Assault Rifle (For the little dudes) And the Shotgun (For the big ones).
When the flood that look like humans come at you try to aim for their big arm. they will die instantly if you shoot it off. it is best to do this at close to medium range with the shotgun. but if you have to you can also use the Assault Rifle.

If you use Grenades try not to use the Plasma Grenades. try to use the Fragmentation grenades because if the flood run at you with the sticky grenade on them you are in trouble.

Also in order to get more check points try to kill all enemies in the area and the comp will generally give you a check point.

Against the big flood just put a shotgun to their chest a pull the trigger it works best that way.

Bloody Warthog:
To start things off lets just beat the game first! Now since the game is beat you should have a little fun. Now go to any level that lets you use the warthog. Now for the good part. When you see an enemy run over him. But it isn't that simple! If your enemy jumps out of the way do a donut and you should get him! Another cool thing is to take a rocket launcher and blast the warthog towards them and it might hit them and they will die!

Silent Level Shortcut:
This will help if your annoyed by always having to go back because the yellow dude locks the door. First find a warthog and get some people inside. Go where the place where there are lots of guys and where kortana says the covenant are putting up a real fight. Go down completely with your warthog and go at full speed directly where the yellow dude is. If you did it right then your warthog will be half way through. Press forward and get out. Then pass the level like you usually do and come back where the warthog is stuck. You may think it is impossible to get out but just press back and then get out. You will be in back of it and now you're done the hard part!

Master sniping technique:
Heres a cheap sniping tactic to aggravate your friends with next time your in a deathmatch.

turn on all vehicles, weapons on normal, and play on the sidewinder level.

grab a sniper rifle and a tank, drive it up to the outer cliff on the blue light side where the rocket launcher is located, drive the tank around the cliff past the rocket launcher towards the outside of the portal.

now back it up the hill next to the portals outside door, the tank should be facing straight at the tree out in the middle of the level.

soon as it's been back up that it cant go any farther (enough that you could get under it) go get a ghost and go off the tank like a ramp.

this is a very stressing trick, it will take a while but you'll know you have gotten in the sniping spot when you fly off the tank out beyond the boundaries of the level.

now you can walk around the whole level and pop out whenever you want and snipe your friends and then pop back out so they cant hit you. (it looks like your flying and sometimes you can keep the ghost up there.