To change your armor, you need to find the skulls. I only got achivements playing in co-op with my little sis. Here is a guide for the skulls:
Skulls
======
When you find a skull, just hold RB until you pick it up, then drop it and
you get credit for finding it. And you must start from the beginning of the
level in order to find any skull, so you can't start from a major checkpoint.
You can get these on any difficulty setting except easy. You can find the
skull and then save and exit if you like to unlock it.
The skulls add points to your meta-game, where you play the main game for
points. So turn them on to up that score. You simply beat the game, get some
skulls, press X when you're choosing a level in the campaign lobby, and then
turn on the score counter along with any skulls for modifiers.
And note that grenade jumping is when you toss a Plasma Grenade to your
feet (not sticking yourself) and then you jump as it explodes to get more air.
I have even done it with a frag, so I know it can be done.
Silver Skulls
-------------
Name: Blind
Reward: Your HUD and gun will go away.
Level: Sierra 117
Location: After the first Phantom drops off enemies in the pool of water,
go straight ahead to the cliff and go to the far right. Look down and drop
to the rock sticking out and pick up the skull.
Name: Grunt Birthday Party
Reward: Confetti thrown for Grunt headshots, and it does just a wee bit damage.
Level: Crow's Nest
Location: In the Drone Sewer before you drop down to the Arbiter, stop before
you fall down the pipe (this is before the Arbiter is in view). Under your
feet is a small space that you can see below the floor. Hug the side and try
to fall so that you land back into that space. Reload your last checkpoint if
you fail.
Name: Cowbell
Reward: Stronger explosions.
Level: The Ark
Location: One of the harder ones to get. You must first get a Grav Lift in
the building near the Scarab battle. Take this lift with you into the building
up the ramp as if you are continuing the mission.
After you fight some enemies in a room with the exploding boxes, the skull
is in the corridor leading downward. At the curve, set the Grav Lift down and
try to make it to the areas above. The skull is on the highest platform, so
if you fail then you need to reload your last checkpoint and keep trying. My
best idea of where to place the lift is somewhere in the middle of the ramp,
toward the bottom, and on the higher ramp of course. If placed right, you
will hop right up there. And for some reason, jumping as you hit the Grav Lift
seems to help.
Name: I Would Have Been Your Daddy
Reward: New dialogue in the game. New dialogue when you play a game with the
skull on. Like the Grunts will say funny stuff.
Level: The Covenant
Location: After Truth is dead, kill all the Flood in the area. The first
area (backtracking now) will be easy. Just try to kill the parasites before
they infect something. The second area is easy with the bots in the sky. And
so is the third area. Hmm, it was actually easier to kill all the Flood rather
than run, provided you had the dual Plasma Rifles.
Now notice the blue rings along the path. Starting from the section near the
elevator, there are two, three, then two. Go into the middle section.
You must jump through the rings along the level in a certain order. The order
is: 4654534. 1 is the ring near start of the bridge, 7 is near Truth. You only
need to use the third set of rings for one, and you don't even need the first
two. Just get to the base of a ring and then jump completely through it. The
rings will all blink on and off if you did it correctly.
The skull appears near the energy bridge that leads to Truth. After you pick
it up, all the rings will glow brightly.
Gold Skulls
-----------
Name: Iron
Reward: Death means you restart the level.
Level: Sierra 117
Location: In the dam area (ha, more dam jokes could be said). It's on the
structure behind the building where you find Johnson. Go right, jump up to
a ledge, follow it all the way left, and the skull is in the corner.
Name: Black Eye
Reward: You must melee friends or foes to recharge you shields.
Level: Crow's Nest
Location: Turn around at the start of the level and go up the stairs. Jump
up to the ledge on the left and then up to the pipe. Follow the pipe to the
skull.
Name: Tough Luck
Reward: Enemies dodge stuff and fight to the death.
Level: Tsavo Highway
Location: After the first shield blocking a tunnel, at the end of the tunnel
is a big pipe on the left. You can jump on some barrels to get on a small
building at your end of the pipe, and from there jump onto the pipe. Follow
it until you hit a fence. Jump on the little metal things sticking out on the
left of the fence and from there carefully jump over the fence. Immediately
on the other side of this fence, look down to the left of the pipe to see a
platform way down below you in the canyon. Either duck under the metal beam or
hop over at the right spot and fall down to that platform. Go to the end of
the small platform and look left to see a skull sitting on a ledge of the
canyon wall. Run and jump over there to get it.
Getting back is tricky, if you want to resume the level. Just jump back across
and follow the platform to the pillar holding the pipe up. Make two precise
jumps on the left side of this pillar to get to the ground below the bridge.
And if you die, you still have the skull so no worry, I hope.
Name: Catch
Reward: Enemies use more grenades.
Level: The Storm
*NOTE: An easy skull if you can grenade jump.*
Location: In the first open field area where you must kill the Wraiths, don't
kill the Wraiths. Kill the first three Ghosts, then the turret gunner on the
AA Wraith, then wait for two more Ghosts to drop in, and kill them. That clears
the field of almost everything but the Wraiths. Contrary to what I've heard,
you can actually kill the AA Wraith (other sources say you shouldn't kill
either), but all I know is that if you kill the normal Wraith on the platform,
that will cause the skull to disappear.
So as for getting the skull, it's tough when you have that normal Wraith
reigning artillery fire upon you. So the first thing you need to do is drive
up the ramp beside it and get on the platform with that Wraith. Kill the
gunner and then do follow the Wraith by sticking close to it so that it doesn't
blast you. Here's the key: stay close and make sure your back is facing the
field. What you want the Wraith to do is make a dash into you, causing you
some damage, and it should blast toward that ramp, but hopefully the Wraith
will get stuck in the gap between the ramp and platform. If done correctly,
the Wraith will be caught in place and unable to get out.
Now is skull time. Go grab a Ghost and take it to the round structure next
to where the AA Wraith was sitting. Notice that the shortest part from the top
of the structure to the ground is facing the platform, hence why we had to
get rid of the Wraith. Back this Ghost up so that the smooth front is facing
that general direction, and park it right next to the structure. Now go to
the ramp at the other end, flip over a downed Warthog, drive it back over to
the other side, and use the Ghost to prop the Hog up. Now jump on the Hog and
get up there to claim your prize.
The whole point of this is to get on top of that structure to claim the skull.
If there is an easier and more sound way of doing this, I have not yet heard
it.
Name: Fog
Reward: No radar.
Level: Floodgate
Location: Returning from the destroyed AA gun, the start of the level, look
up to the right when you hit the ramp where you picked up the turret. Keep
looking until a Flood pops up. Immediately shoot him and hope that the skull
flies forward and down to you. Otherwise, quickly revert to your last save,
which was just a moment ago, and try again. Please, go to save and quit in
the pause menu because no one wants to play this mission again, unless you
have to.
Name: Famine
Reward: Less ammo in enemy guns you pick up.
Level: The Ark
*NOTE: Another one of those easy-to-get skulls if you can grenade jump.*
Location: This is a mission-halting skull right here, because you might be at
this one for a while. The skull is located on a bridge adjacent to the wall
entrance. That means that the place where you take the tanks to cross over
the wall, to the right of that. It's a small bridge that isn't connected to
anything, but you'll go under it on your way to the frigate (where all the
tanks unload) and you'll cross back under on your way to the wall.
The skull itself is located at the very tip of the last support thing that
hangs over the ground; not at the top, at the tip. It will be sitting in a
little crevice that you cannot reach by normal means. Enter frustration.
Actually, I took care of the frustration. But the easiest way I heard about
was using a co-op teammate to jump off of. That sounds fine, but there is an
alernate way for you lone wolves out there.
Up the path a bit is a circular area where you should blast some enemies. After
they are dead, look to the rock ledge that looks over the way you came. If you
try to walk into that or take a vehicle over the smooth surface, you hit an
invisible wall. But, there is a way around that wall.
Take a Ghost and follow this "wall" to the raised tip of the rock (the part
that overlooks your path). If you feel out the wall you will eventually find
a spot where you can get around it. Remember that spot and take your Ghost
around the slanted edge to the end of the bridge with the skull. But you can't
get the Ghost up there. That's not the goal. Park the Ghost against the rail
of the bridge, get out, and locate a Chopper (either near the fight area, or
at the other wall area). Take the Chopper to the Ghost using the same path and
park it so that its front and the Ghost's front are facing opposite directions,
but be sure they are as close as possible.
The point of the Chopper and the Ghost was to create a ramp for a Prowler.
Hopefully you have the one from the start of this driving segment, but sadly
you must kill any riders; kill them quick or they will be on to you. Take the
Prowler up to the "ramp" and use it to get the big vehicle onto the bridge.
Now for the fun part. Take the Prowler over to the support structure and park
it on either the left or right side, with just the tip of it hanging over the
slope, but you better leave most of the weight on the flat surface. Jump onto
the turret and from there jump to the platform with the skull and that's that.
If you miss and fall you can climb back up by either the opening by the wall
or by the inclining rock along your path, and then try again. Good stuff.
Name: Thunderstorm
Reward: All enemies are stronger (improved ranks).
Level: The Covenant
Location: When you get the Hornet, fly to the tower that the Arbiter disabled
and find the skull in the balcony overlooking the sea. It's just up a small
ramp. You will also find a terminal in the area.
Name: Tilt
Reward: Enemy armor is stronger.
Level: Cortana
Location: In the second big room where you fight a bunch of Flood. It's also
where you find an access terminal that links to Cortana.
After you clear the room of Flood (don't worry about the tiny constant trickle
from the back), go to the left side of the room and find the dirt-like ledges.
If you look up, you will see that the lower set of ledges on the wall will
lead to another set to the right a bit higher up. You need to get to that top
set of ledges to reach the skull.
You do this by standing on a small, round, purple thing that sticks directly
under both sets of ledges. Have you found it? Good. Now jump on the blue
spot on top of it. The only way you reach the lowest ledge is by jumping onto
the protrusion from this point. I mean the smooth surface above the blue spot.
You jump on that, but in a split second you must jump up to the ledge on the
left. You have to land on the left side of this smooth thing and quickly jump,
or else you'll fall off or jump in the wrong direction. Trust me, you'll know
what I'm talking about after you've jumped about twenty times and missed.
Again, you're not jumping from the blue spot, you're jumping from the shield-
like thing next to it that sticks up.
Now that you're on the ledge, look to the wall and look left. You must jump
up to the slightly higher ledge above the one you're on. You do this by
standing at the very edge of the surface and jumping. If you aren't out far
enough, you jump into the curved wall and end up back on the ground.
On the second ledge, now you must jump over the gap and onto the highest set
of ledges, which is really just two very close together. Again, get to the very
edge of the surface you stand on, as far as you can get from the wall, and then
jump up to the upper ledges at their point farthest from the wall.
I know, hard to explain. You just have to get there to know what I'm talking
about. The only issue once you're on the ledges is to avoid jumping into the
wall that curves. Mind that rule and you should make it up in no time.
The skull itself is across the beam-like thing and among some bones on the
purple surface. Trust me, it's a lot easier done than said.
Name: Mythic
Reward: Harder to kill enemies.
Level: Halo
Location: Follow the right rock wall at the start of the level to the skull.
In the first rock tunnel you should find a small path leading you to the final
prize. Sweet mercy, at last!