Leave level boundary in Valhalla (Beta version) - There is a beach at the end of one side of the map. At the far left of the beach, there is at least one small rock and a big rock. Keep trying to jump on the big rock to get on it. Once there, you should see a gap.

Try to go through the gap by approaching it standing up. Once you are out far enough, walk through while crouching, and you should be out of the boundary. Note: This may require take several attempts.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Unlockables

Askar (10 points) Score over points in the Campaign meta-game on the fourth mission.
Assault (30 points) Finish the fourth mission of the Campaign on Normal, Heroic, or Legendary.

Black Eye (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.

Campaign Complete: Heroic (125 points) Finish the Campaign on Heroic (unlocks achievement for Normal, if not earned).
Campaign Complete: Legendary (125 points) Finish the Campaign on Legendary (unlocks achievements for Normal and Heroic, if not earned).
Campaign Complete: Normal (125 points) Finish the Campaign on Normal.
Catch (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
Cavalier (10 points) Score over points in the Campaign meta-game on the third mission.
Cleansing (30 points) Finish the fifth mission of the Campaign on Normal, Heroic, or Legendary.

Demon (10 points) Score over points in the Campaign meta-game on the second mission.

Exterminator (10 points) Score over points in the Campaign meta-game on the fifth mission.

Famine (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
Fear the Pink Mist (5 points) Kill 5 enemies with the needler in a ranked free for all playlist or in campaign.
Fog (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.

Guerilla (10 points) Score over points in the Campaign meta-game on the first mission.

Headshot Honcho (5 points) Kill 10 enemies with headshots in a ranked free for all playlist or in campaign.
Holdout (20 points) Finish the second mission of the Campaign on Normal, Heroic, or Legendary.

Iron (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.

Killing Frenzy (5 points) Kill 10 enemies without dying in any ranked free for all playlist (Online).

Landfall (20 points) Finish the first mission of the Campaign on Normal, Heroic, or Legendary.
Last Stand (40 points) Finish the seventh mission of the Campaign on Normal, Heroic, or Legendary.
Lee R Wilson Memorial (5 points) Score 5 grenade sticks in any ranked free for all playlist (Online).

Marathon Man (40 points) Locate and access all Terminals in the Campaign.
Maybe Next Time Buddy (5 points) Board the same vehicle within 10 seconds after being boarded in any free for all playlist (Online).
Mongoose Mowdown (5 points) Splatter an enemy with the mongoose in a ranked free for all playlist (Online).
MVP (5 points) Earn the MVP in any ranked playlist (Online).
Mythic (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.

Orpheus (10 points) Score over points in the Campaign meta-game on eighth mission.
Overkill (5 points) Kill 4 enemies within 4 seconds of one another in a ranked free for all playlist (Online).

Ranger (10 points) Score over points in the Campaign meta-game on the sixth mission.
Reclaimer (10 points) Score over points in the Campaign meta-game on the final mission.
Refuge (30 points) Finish the sixth mission of the Campaign on Normal, Heroic, or Legendary.
Return (50 points) Finish the final mission of the Campaign on Normal, Heroic, or Legendary.

Spartan Officer (25 points) Advance to the Spartan Officer ranks (Online).
Spartan Program Recruit (10 points) Win your first game in matchmaking to sign up for the Spartan Program (Online).
Steppin' Razor (5 points) Score a Triple Kill with the sword in a ranked free for all playlist (Online).

The Key (40 points) Finish the eighth mission of the Campaign on Normal, Heroic, or Legendary.
The Road (20 points) Finish the third mission of the Campaign on Normal, Heroic, or Legendary.
Thunderstorm (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
Tilt (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
Too Close to the Sun (5 points) Destroy an enemy banshee with the spartan laser or missile pod in a ranked playlist or in campaign.
Tough Luck (10 points) Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
Triple Kill (5 points) Kill 3 enemies within 4 seconds of one another in a ranked free for all playlist (Online).
Two for One (5 points) Score a Double Kill with a single spartan laser shot in a ranked free for all playlist (Online).

UNSC Spartan (15 points) Earn your Sergeant rating to be recognized as a true Spartan (Online).
Up Close and Personal (5 points) Kill 5 enemies by melee or assassination in a ranked free for all playlist (Online).
Used Car Salesman (5 points) Destroy a vehicle that has three enemies in it in a ranked playlist or in campaign.

Vanguard (10 points) Score over points in the Campaign meta-game on the seventh mission.

We're in for some Chop (5 points) Destroy an enemy vehicle with equipment in a ranked playlist or in campaign.

Armor parts - Complete the indicated task in Campaign mode to unlock the corresponding armor part in Multi-player mode.

Elite Ascetic Body Earn the "Up Close and Personal" achievement.
Elite Ascetic Head Earn the "Steppin' Razor" achievement.
Elite Ascetic Shoulders Earn the "Overkill" achievement.

Elite Commando Body Earn the "Triple Kill" achievement.
Elite Commando Head Earn the "Overkill" achievement.
Elite Commando Shoulders Earn the "Killing Frenzy" achievement.

Elite Flight Body Complete "Tsavo Highway" under the Heroic or Legendary difficulty.
Elite Flight Head Complete Campaign mode under the Heroic difficulty.
Elite Flight Shoulders Complete "The Ark" under the Heroic or higher difficulty.

Spartan EOD Body Complete "Tsavo Highway" under the Legendary difficulty.
Spartan EOD Head Complete Campaign mode under the Legendary difficulty.
Spartan EOD Shoulders Complete "The Ark" under the Legendary difficulty.

Spartan EVA Body Complete "Tsavo Highway" under the Normal or higher difficulty.
Spartan EVA Head Complete Campaign mode under the Normal or higher difficulty.
Spartan EVA Shoulders Complete "The Ark" under the Normal or higher difficulty.

Spartan Mark V Head Earn the "UNSC Spartan" achievement.
Spartan ODST Head Earn the "Spartan Graduate" achievement.
Spartan Rogue Head Earn the "Spartan Officer" achievement.

Spartan Scout Body Earn the "Too Close to the Sun" achievement.
Spartan Scout Head Earn the "Used Car Salesman" achievement.
Spartan Scout Shoulders Earn the "We're in for Some Chop" achievement.
Spartan Security Head Earn 1,000 Gamerscore points.
Spartan Security Shoulders Earn 250 Gamerscore points.

Alternate ending - Complete the final mission under the Legendary difficulty setting.

Hayabusa Armor - Collect the hidden skulls throughout the game to unlock the pieces of Hayabusa armor.

Hayabusa Chest Collect 6 hidden skulls
Hayabusa Helmet Collect 13 hidden skulls
Hayabusa Shoulders Collect 9 hidden skulls

Black Eye Skull - From the start of the "Crow's Nest" mission, without moving anything, you can see a massive pipe above you. Walk up the stairs, then up the next set. Turn to your left and jump up on the shelf unit. From there, jump to the red cross beam then up onto the pipe. From there, just walk to the end. Walk over the Black Eye Skull to claim it. You do not have to pick this skull up, it does it automatically. Its description reads "Bash your way to better health." Your shield will not auto-recharge anymore, and you must kill an enemy with a melee attack.

Blind Skull - In the first mission, get to the area where you see the Phantom overhead. A Marine will point it out. Jump over the rocks and follow the path on the right. When you reach the cliff, there will be a rock over the side. The skull is at the end of the rock. Its description reads "Shoot from the hip". This skull must be activated before you start a Campaign map.

Catch Skull - In "The Storm" mission, do a grenade jump off a Warthog; or get a Ghost and go back to the ramp from which you first appeared. Use the Ghost to push a big square crate over to the front of the silo pod that has the skull. Jump from the box to reach the skull. Its description reads "Pull pin. Count to three. Throw." All NPCs will have two grenades and will throw them frequently.

Cowbell Skull (Level: The Ark) - First pick up a grav lift from the small building near where you fight the scarab. Now proceed through the level until you reach the second sloping hallway (stairway), you should see some partitioned risers (platforms) halfway down. The skull is on the top level. Toss the grav-lift on the right side of the hall so it lands on the fourth little green dot from the door. Then run, jump, and use the grav-lift to propel you to the top. You should reach a ckeckpoint just as you enter the room so it you miss just try again.

Famine Skull (The Ark) - In "The Ark" mission, enter the valley to the right after the wrecked Phantom. Look to the left to find a large boulder. Use a Ghost to reach the side of the boulder that is closest to the bridge overhead. Get the Ghost up the side of the wall by using the thrust. Get two Ghosts up the wall to the top of the bridge. Stack them on top of each other next to the beam where the skull is located. Then, jump from the top of the Ghosts towards the skull and land on the beam. Its description reads "Trust us. Bring a magazine." Dropped weapons will have less ammunition.

Fog Skull - In the "Floodgate" mission, while you are walking down from the anti-air gun destroyed in the last mission, you will see a ramp next to a missile launcher. Near this ramp is a checkpoint. After reaching it, you will hear a Marine yell, "There! Over there!". Look up and to the right at the roof of the building next to the missile launcher. There is a lone Flood that is holding the skull. Kill him before he jumps. He will drop the skull on the ground. Note: If you shoot too early the skull will get stuck on the roof. Its description reads "You'll miss those eyes in the back of your head." You will no longer have a motion sensor.

Grunt Birthday Party Skull - Immediately after the first objective of the "Crow's Nest" mission, while en route to the barracks you will fall down to a pipe. Get to the end that pipe. Look over the edge to find a small space a few feet below. Drop over then quickly pull back to land under the floor you dropped from. The skull will be at the end of this area. When this skull is active, headshots will cause enemies to explode.

Iron Skull - After freeing Johnson and killing all the Covenant that come in the Phantoms in the "Sierra 117" mission, walk to the Pelican that arrives to pick you up. Face the cargo bay, but instead of hopping in turn left and follow the path. Once you hit the wall, jump up to your right to eventually reach a walkway. Walk all the way to the end. Take a left, then a right. At the end of this a little bit to the right is a dark corner. Press Up to turn your light on and you should see the skull. If you cannot see it you are probably standing on it. Its description reads "Death carries a heavy price...". Instead of only losing 25 points when you die, you will lose 75.

Mythic Skull - Follow the normal path in the "Halo" mission until you reach a cave. Hug the right wall until it branches off into a small path. At the end of this path is some rubble, and in front is the skull. Its description reads "Coverage under the Covenant health plan." It gives a 3x score multiplier for all kills.

Smarter respawning - When playing in Co-op under the Legendary difficulty setting, it is best to have one player keep their distance from the enemy at all times. By doing this, if the rest of the players die they will respawn around the player that is farther back, rather than having to restart the checkpoint.

Sierra 117: Monkeys with teddy bear - After you come into contact with the snipers in the woods, follow along the left rock path. You will find a strange family of monkeys holding a teddy bear.

Skulls - You can only claim/collect the skulls under the Normal, Heroic or Legendary difficulty settings. You must start from the beginning of the mission. You will not be able to see the skulls if you load from a checkpoint. In Co-op mode, as soon as one player picks the skull up, all players will receive the achievement. Not all skulls earn an achievement; only nine of them do so. To use skulls, press X at the lobby screen. This will give you a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements.

Thunderstorm Skull - In "The Covenant" mission, shut down Tower 1 and get access to the Hornet. Fly to Tower 2 (which is the one that the Arbiter shut down). Go up the stairs. Go to the middle part that connects both. There should be a stairway that leads up to a platform where the skull is found. Its description reads "Field promotions for everyone!" Enemies will be stronger.

Tilt Skull - Mission: Cortana - When in the circular type room with all the flood look for a small structure piece next to two archways, jump on top if it and up on the rocks to the top left, turn left and jump up again, then do a 180 and jump to the rocks across from you. After that follow the the rock sticking out and leading up on top of of the original circular room, the skull will be in a pile of blood.

Tough Luck Skull - On Tsavo Highway, about halfway through the mission (right after you are forced to walk through a large blue barrier), you will come out of a tunnel on the highway, and see a large pipeline on your left. Drop down in between the two, and run to the wall in front of you. Follow the wall all the way to where it connects with the cliff on your right, and turn to the left. There should be a few ledges--simply crouch jump from ledge to ledge, and the last one should have the "Tough Luck" skull on it.

Infinite Spartan laser in Halo mission - Use the following trick to get a Spartan laser with unlimited ammunition in the "Halo" mission. Play through the level until you meet up with Sergeant Johnson. Shoot him until he falls down and drops the Spartan laser. Quickly pick it up, and shoot it at him. Keep shooting him until he kills you. When you respawn, knock him down again, and pick up his laser. He should get back up with another laser on his back. When he does, shoot him down again, and pick up the new gun he drops. This laser should now have unlimited Spartan ammunition. This makes it easier to destroy the Guilty Spark 343.