On the last level, right when you start it, if you look behind you there are some weapons next to the Pelican.  These are extremely useful for fighting the flood.  (There are shotguns, rockets, etc.)

On the first level, there is some little secret caveman dolls.  They spurt out blood when you shoot them.

On the last level, (Halo) at the very end, when you drive the warthog to the ship, there is a hidden grunt behind some rocks.  He will say things like "The jerk store called, and they’re all out of you!"  It is very funny for the first time.  If you want to see where the grunt is located, then simply look it up in google, there is several videos on it.

You can only collect Skulls on the Normal, Heroic, and Legendary difficulty setting. 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 if there is one. Not all Skulls earn an achievement; only nine of them do. To use Skulls, press X at the lobby screen. This will give you a bonus multiplier for Co-op mode and go towards earning the Guerrilla, Demon, etc. achievements.



Black Eye Skull: At the start of the "Crow’s Nest" mission, without moving anything, you can see a huge pipe above you. Walk up the stairs, then up the next set. Turn to your left and jump up on the shelf unit. Jump to the red cross beam, then up onto the pipe. Then, just walk to the end. Walk over the Black Eye Skull to collect it. You do not have to pick this Skull up, it does it automatically. The Skull description reads "Bash your way to better health." When this Skull is active, your shield will instantly charge when you use melee attacks, and will not auto-recharge anymore.



Blind Skull: In the "Sierra 117" mission, when you get to the area where you see the Phantom overhead (one of the marines will point it out), jump over the rocks, and keep following the path on the right. When you get to the cliff, there will be a rock over the side. The Skull is on the end of the rock. The Skull description reads "Shoot from the hip." Note: This Skull has to be activated before you start a campaign map.



Catch Skull: In "The Storm" mission, go to where the first anti-air Wraith tank is located. There is a large gray cylinder in the that area on the ground on the far side from where you enter. The Skull is on the top of the silo. To get up there, perform a grenade jump off a Warthog or use a vehicle to push a large crate to the side of it; jump on the crate, then the silo. Note: Do not destroy the Wraith near the door or the Skull will disappear. The Skull description reads "Pull pin. Count to three. Throw." When this Skull is active, every NPC will carry two grenades and throws them often.



Cowbell Skull: In "The Ark" mission, pick up a Grav Lift from the small building near where you fight the Scarab. Then, 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. Throw 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 checkpoint just as you enter the room, so if you miss you can try again. When this Skull is active, explosive force is increased.



Famine Skull: In "The Ark" mission, when you first go into the valley to the right after the wrecked Phantom, look to your left to see a huge boulder. Use a Ghost to get to the side of the boulder that is closest to the bridge overhead. It is easy to pilot the Ghost up the side of the wall using the thrust. To get the Skull, pilot two Ghosts up the wall to the top of the bridge and stack them on top of one another next to the beam where the Skull is placed. Then, jump from the top of the Ghosts towards the Skull and land on the beam. The Skull description reads "Trust us. Bring a magazine." When this Skull is active, dropped weapons will have very little ammunition compared to normal.



Fog Skull: In the "Floodgate" mission, as you are walking down from the anti-air gun you destroyed in the previous mission, there will be a ramp (next to a missile launcher). Around this ramp, there is a checkpoint. At this point, you should also hear a marine yelling, “There! Over There!”. Look up and to the right, directly at the roof of the building next to the missile launcher. There is a Flood form (not to be mistaken with the two other Flood forms jumping in front of you), which is holding the Skull. Kill him before he jumps, and he will drop the Skull down to the ground where you can retrieve it. Note: Correct timing is required because if you kill him too early, the Skull will get stuck on the roof. The Skull description reads "You’ll miss those eyes in the back of your head." When this Skull is active, your motion sensor disappears.



Grunt Birthday Party Skull: In the "Crow’s Nest" mission, immediately after the first objective, while en route to the barracks, you will fall down to a pipe. Go to the end of that pipe, then 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 is at the end of this area. When this Skull is active, it makes Grunts explode in confetti when they are headshot. There will also be some cheering, like a birthday party.



Iron Skull: In the "Sierra 117" mission, after freeing Johnson and killing all the Covenant that come in the Phantoms, walk to the Pelican that arrives to pick you up. Face the cargo bay, but instead of going 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. The Skull description reads "Death carries a heavy price...". When this Skull is active, instead of losing 25 points when you die, you will lose 75.



IWHBYD Skull: In "The Covenant" mission, go to the last tower. Before you get to the Prophet of Truth, there are a series of bridges with Halos. Do not jump through any yet. First, kill the Prophet of Truth, and defeat the bridges of enemies. Then, jump through the Halos in this order, with "1" being at the entrance and "7" being the end near where the Prophet of Truth was located: 4, 5, 6, 5, 7, 6, 5, 4, 1, 2, 3, 2, 7, 2, 1, 3, 4, 6, 7, 5, 4, 6, 5, 4, 5, 3. Alternately, jump through the Halos in the following order: 1, 3, 4, 6, 7, 5, 4, 6, 5, 4, 5, 3, 4. This is much easier in Co-op mode with three or four people. This is the Halo theme if it was played on a phone by dialing the numbers. If done correctly, the Halos will start flashing, and the Skull will appear near the Prophet’s ramp. If the Halos do not flash, try again. The Skull description reads "But the dog beat me over the fence." When this Skull is active, there is bonus dialogue throughout the game. IWHBYD stands for "I Would Have Been Your Daddy", which is a level chapter from the original Halo. -From: crimsontwist



Mythic Skull: In the "Halo" mission, follow the normal path until you reach a cave. Follow 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. The Skull description reads "Coverage under the Covenant health plan." When this Skull is active, you will get a 3x score multiplier for all kills.



Thunderstorm Skull: In "The Covenant" mission, after you shut down the first tower and get access to the Hornet, fly to the second tower (the one the Arbiter shut down). While walking up the stairs, go to the middle part that connects both, and there should be stairs leading up to a platform where the Skull is. The Skull description reads "Field promotions for everyone!" When this Skull is active, enemies will be much stronger.



Tilt Skull: In the "Cortana" mission, 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, then go up on the rocks to the top left. Turn left and jump up again, then turn 180 degrees and jump to the rocks across from you. Then, follow the rock sticking out and leading up on top of the original circular room. The Skull is in a pool of blood. The Skull description reads "What was once resistant is now immunity." When this Skull is active, enemies will have different body parts that are resistance to certain bullet types.



Tough Luck Skull: In the "Tsavo Highway" mission, once you get past the part where you are forced to leave your vehicle, you will go through a tunnel and onto an open freeway. Immediately after coming out of the tunnel, you will see a huge pipe with two yellow ladders. Climb the second ladder. You should see a Regenerator power-up. Proceed to your right onto a small platform ledge where the yellow chain link fence is located. Jump onto that ledge, crossing over the fence. Directly behind you should be a large column that is holding up the giant pipe you were just on. Below that is a walkway. Fall off the ledge onto the walkway. Walk to the end of that. You should see a hidden ledge with the Skull on it. A few attempts may be requires to jump across. The Skull description reads "Your foes always make every saving throw."

Hold X and press a b y y a b b. Only works on Heroic

Un-Verified

This cheat is rather hard and time consuming but if you love the energy sword It might be worth it.



From the beginning of the level head down the stairs you are soppossed to. Turn either left or right and you will see an inning. Behind it is a small space under the stairway, which is extremely important and you must remember where it is. From there, go back up the stairs and find a marine with either a battle rifle or a pistol, then kill them and take their weapon. Now you can start the cheat.



First, always remember to stay FAR AWAY from the Arbiter. One blast from his gun will instantly kill you, and you will be shot at alot by him. Once out of the view of the Arbiter, (where he cant see you,) start killing marines by using quick headshots made easy by the acuracy of the battle rifle or the pistol. (DO NOT GO PAST THE MARINE STANDING NEXT TO THE DOOR TO THE NEXT AREA, THE DOORS WILL LOCK YOU OUT) After killing about 4-5 armed marines, the others will begin shooting at you. Once they start shooting at you, run down the stairs and under them to the secret hiding spot under the stairs. They will follow you down the stairs, but will not enter that area, and soon will go back to normal. Repeat this including all the staff as well, and eventually kill everyone except Johnson, Miranda Keyes, The Arbiter, and Miranda’s Guard. Then, use headshots to kill the Arbiter, and if Johnson shoots you, run back to your hiding spot. (You cannot completely kill him or the Arbiter, they come back to life.) Kill the Arbiter four to five times without stopping and on the fourth or fifth time he falls to the ground, he will also drop the energy sword. You can then pick it up and carry it into the hanger with you, and use it for the rest of the level. As an extra bonus, If You kill the Arbiter once or twice again, AFTER he gives you the sword, he will also drop his plasma rifle. (Note: You may have to kill him multiple times to get the plasma rifle, it depends how fast your game is running and how quickly he respawns.)



IMPORTANT: Do not try killing the arbiter without killing everyone else first, The Arbiter will shoot you once and you will die instantly or a barage of well armed marines will take you down in seconds.



IMPORTANT(2): Do not kill anyone past the man at the door at the bottom of the stairs, even if you run when they shoot at you, there will be too many on the stairs when you come back the way you came, and you’ll be killed and have to start over, and yes, if you die you have to start the cheat over.

One the first level, when you can see the sea, go all the way to the right and then down. There is a small edge and a skull.

on the level were u r underground and the woman is speacking on the big screens allover the room u can jump on to a ledge thn another and thn the pipe (the ledges are on the far side from the screens) walk on the pipe and there is a scull up there im not shure what it does but the achivment name is "blackeye" note: the ledges are right by the pipe its very simple and can take as much time needed