Edit: 343 decided the achievements weren't hard enough so they decreased all par scores based on peoples performances on the missions, and laughed at everyone including myself getting par scores of 25k + (half Halo 1, ALL Halo 2). If you haven't done any yet, congrats you get it much easier then the first people to attempt par scores. For this mission, you need 10,000 instead of 25,000.
This mission is quite easy, and the best part is you can sit comfortably in a scorpion(tank) through-out most of the mission.
Difficulty: Heroic ( probably can be done on normal now)
Skulls: Anger, Catch, Famine, Fog, Iron, Thats Just... Wrong.
Note: If you die you will want to save/quit then resume campaign.
In the beginning, run up the hill, grenade the 3 grunts in front of you and head in their direction. Now what you are going to want to do is use your rocket to kill the remaining 3-4 turrets, after that run up the right ramp and charge the right side of the building. Make sure to kill the sniper if hes there avoiding his shots. Walk up into the building and there should be an immediate sniper on the ground full of ammunition, grab it and then head left from where you were facing the sniper rifle, jump where the turret was, then again above where you came from. There will be some frags up there and some rocket ammo.
After grabbing the Rocket and frags immediately jump down and in the direction where you are supposed to go (right of the building). Going in this direction there will be 3 snipers after 5-10 seconds of walking, make sure to kill them quickly and continue forward.
Once the next building that you must go into is in view, snipe all enemies in your site, start with the elites, headshot them. Then take out the turrets (randomly placed every time). Heading down the hill towards the building use your rockets to kill any ghosts coming at you and frag the jackals. Carefully enter the building and into the room you are supposed to go in, try to stick the elites, if you are out of sniper ammo just grab a dead enemies gun outside before entering. Once the room is cleared activate the bridge and head out towards you new scorpion that's being brought in by the pelican.
With the scorpion you will want to take your time, but don't look around every corner for enemies, just keep pushing forward killing everything. Do know that after you kill the 2 wraiths and cross the bridge you will have 4 ghosts come out from where you are supposed to go and 2 banshees will attack you from behind so try to kill them quickly to not take too much damage.
After getting to the end part with the tank, get the tank as close as you can to the entrance that the scorpion cannot access. Enemies will keep coming out of the structure so shoot and get a ton of instant kills. If they stop coming go forward to spawn another group, get back in your scorpion and fire away. When they stop coming proceed forward.
When entering the next area make sure to get a BR from one of the marines following you, and plasma grenade any elites, your plasma's will be your best weapon in this level.
When you get to the sniper spot (on top the waterfall) you will want to head left, try killing some enemies with the sniper first, and head left into a tunnel with what ever weapon you have. Kill the elite inside, and shoot any snipers at the exit (careful not to get shot). As you head to the left where the exit is, if you need to use a rocket, but expend all your grenades first.
When you get to a open area with a ton of structures you are at the end, just watch out for 2 snipers on top of where you are supposed to go, and prepare for some elite guards inside. Use plasma's, sniper, or if you have rocket ammo this is the spot to use it.
Good luck and have fun, this level was easy in my opinion so just follow my method to get this achievement.