Kill an Enemy and Hijack His Vehicle:
This is a fairly complicated skill but if it is done successfully it can prove very effective. First get a sword and when an enemy drives at you jump before you are hit. While you're in the air, face down and use R trigger attack to kill the driver. If you're quick enough, you can press X and get into the gunner seat on a warthog or even the driver seat on other vehicles.

'Unlimited' Energy Sword:
On the level, The Arbiter, deplete your energy sword, keep it with you, and complete the level. When the next level starts, draw your sword. It won't have a blade, but it'll work just fine. Voila!

Get on Top of Tower on Lockout:
Make sure you set it so everyone has rockets and swords (I also recammend giving everyone overshields as someone is almost guaranteed to be hit by the sword). Have a person go the spot where the sniper rifle is located and start jumping up and down. Stand on the main center platform and a little bit under the jumping person. Do the flying sword trick (as seen below) at the jumping person when they are at their peak of the jump. If done correctly you will shoot to the top of the tower. If you want everybody on top of the tower then you will want overshields. As soon as you get the first person to the top of the tower, everybody else and use the flying sword technique to shoot themselves up there. Then have everyone try to kill eachother and last one alive is King of the Hill.

Iron mode:
In a Legendary game, get to Master Chief's last level and look up and grab the skull that passes by in the gravity lift.

Envy mode:
In a Legendary game, get to the Pelican Landing zone in the Delta Halo level. Find the door obscured by rock near one of the plasma turrets. Jump on the rock in front of the door, then jump to the ledge immediately above the door, then jump up onto the ledge to the left. Now use grenades to boost yourself on top of the building and take the skull for Active Camouflage.

Angry mode:
You're Master Chief, you're in the Holy City, and your mission is to get the index from the Prophet of Truth. When you come to an outdoor area, go right and run past the gravity lift to a wall. Climb the dirt mound to the left to get on top of the wall, then follow the wall to the secret skull.

Grunt birthday party skull:
Play the Arbiter level on legendary normally until you get to the part with the banshee. Fly the banshee strait down, and in the lower-middle of the level fly aroung until you see some grunts dancing in a circle. They're dancing around a skull that will make a shield-less enemy explode if you headshot him.

Armory Skull:
Start The Armory on legendary. At some point, Sarge will show up in a little tram. Don't get in until he says "Would it help if I said please?" When you start moving face the right side. Hold X to get the skull. Enemies will always know where you are.

Cairo skull:
In Priority Shift, on Legendary, there are two short platforms in the second room with a bunch of potted plants. One of them will have glass windows and an area you can get up to. So, either jump on the glass, or onto the metal frame. Crouch jump up to the slanted beam and look for a really big beam that's supported by a bunch of smaller beams. Get on one of the smaller ones and crouch jump up to the main one. Walk along the beam towards the platform and the window. There should be a trashcan on the platform with a skull in it. All enemies will be promoted to the maximum rank.

Metropolis skull:
When you leave the sewers in Metropolis (on Legendary), you'll be in a field. Head to the middle platform, and grenade jump onto the ornate looking beam. At the top, go to the left tower and walk around its ledge to the left for a skull. Enemies will always drop two grenades.

Outskirts skull:
At the beginning of the level go through the first door, turn around, and jump onto the light above the door. Then, with your back to the wall, crouch jump onto the building to the left. Follow the hallway to the skull. Disables your HUD.

The Oracle skull:
In the Oracle stage on Legendary, you'll come to a room that overlooks a fight between flood and heretics. Break the glass, jump down into the room, and then break the glass on the other side and grenade jump up into that room. A skull should be on the left side of that room. Any weapon you get will have half its normal ammo.

Regret skull:
In Regret on Legendary, travel on the gondola and head into the building that's right there when you get off. Jump on the large, stone block inside, and then jump onto the only available ledge. Follow the ledge outside and go to the left. Jump up to the next ledge. Do whatever you need to do in order to make progress
along the outside of the building. You'll eventually happen upon a small
grassy area with two dancing-cloaked-sword Elites. Jump down and get the skull.
All enemies will be invisible.

Quarantine Zone skull:
On Legendary, turn and go through the tunnel right at the beginning of the level. Head along the left side of the field to the pit, and walk along the ledge that surrounds the pit to the skull. Anything that is knocked into the air will fly much farther.

The Great Journey skull:
In the Great Journey, on Legendary, you'll have to give Sarge air cover with your Banshee. When you get to the end of the path, land your Banshee on a ledge above the door Sarge blew a hole in, and get out. Walk on the ledge to the left of the door. Look for an ornate stone and jump on it. Walk to the top, and head to the back of the structure. Cross the large dirt area, and up the big beam. Grab the skull. You shields will no longer recharge unless you whack something with a melee weapon.

Scarab Gun:
On the Metropolis level, play cooperatively. Cross the bridge at the beginning of the level, and kill everything you come across be it friend or foe. Eventually, Cortana will mention a "Welcoming party." A bunch of stuff will show up; destroy all the wraiths but only one of the banshees. One player should then go through the tunnel while the other player keeps the banshees distracted. When player one arrives at the circular opening, he should kill the jackal and then head back to player 2. Now try and get the banshee to follow you both into the tunnel. Both players should stand near the circular opening to lure the banshee into that room. It will get stuck on the entrance, so shoot off its wings. One player should walk in front of the banshee, while the other pushes it. Take it to the left where there is a checkpoint. As soon as you reach the checkpoint, the elite will disappear, and you will have a short window during which you can board the banshee. Fly outside and up, and look for two sky scrapers with bridges connecting them. The scarab gun is on one of the bridges.

The Flying Sword Technique:
This technique makes you fly to a person you want to kill with a sword across the stage.
Materials needed: Get a rocket launcher and a sword. Aim with the rocket launcher with the sword in the secondary slot.
Aim the rocket launcher in either scope or regular, depending if the person is far away or in mid range.
When you aim for the person, quickly press the Y button and prior to pressing the Y button, double pump the trigger to initiate the sword attack. If you do it right, you will launch yourself towards the person that you aimed for. That doesn't mean you automatically hit them. If they side step they will avoid the attack and you are left with a sword and a rocket launcher with no rockets. Regardless this is a good way to get to the other side of the stage if you're playing capture the flag and the range of this attack is infinite. If you have mastered the technique, you will be able to go the whole distance of waterworks without breaking a sweat!

Side Note: It's better to empty out the rocket launcher just in case you mess up by giving away your position due to a misfired rocket.

Side note II: This will take awhile to master because you have to time the buttons perfectly. It is especially hard to master in combat because there isnt time to do everything right. The preferred method of practice is if you do it to a unsuspecting sniper or a noob that gets scared if someone is coming after him or her.

The Proper Sniper / Rocket Transfer (Flying Sword Technique):
The best way to perform the transfer (either zoomed in or not) is to slide your thumb from the "Y" button to the "X", then right when your thumb hits the "X" button, press right trigger instantly so you almost hit "X" and right trigger at the same time. Once timing is right you can hit it on your first try every time.