As soon as you get off the pelican dropship, proceed forward until you hit a path. I recommend staying back and using your sniper rifle to take out as many covenant as you can. Leave the grunts until you go out and fight.
Once you have cleared that area proceed up the path on the far left to where you entered, beside the turret. Once you move up this path there should be a little area with a cliff or rock wall in front of you. There should be a grunt in a turret above, take him out. Once you do that covenant should come pouring from both sides. Clear the area and move up the path to the left. Snipe the elite on the tall cliff edge. You can either walk all the way around the cliff and fight two Jackals or you can go immediatly up into the action. I prefer to go all the way around so you have more distance to fight in. Your troops should come up and help once you get in the battle. After a while of clearing the area with the large valley, the covenant will get reinforcments. Grab the invisibilty and throw a grenade at the covenant dropping out of the dropship (especially the elite). After elimating all the covenant, go down the valley and through the path. More covenant will come and challenge you. Fight them off. You should be in a large area with the gravity lift in the center. Eliminate the covenant in the turrets and proceed into the area. There will be a few loads of the covenant and the final will be Hunters. If your running short of troops, they will call in a pelican dropship to bring reinforcements.
After you clear the area and finish off the Hunters, stand on the gravity lift. It should pull you up to the ship. Once in the ship, you will be dropped off in a small room. Cavenant come pouring from all doors. Make sure you kill the elites fast. Clear the room and proceed through the door that the hunters came through. There will be a locked door down the path. Go back up and into another door. Proceed through the halls and eliminate all the covenant you see. You should run into a big room with about three levels. You have to fight off the covenant and let your troops through the door. Once you have assembled you troops, continue to go look for the Captain. After a while of walking through the halls, eliminating the covenant, you should come across a room filled with jail cells. There's two of them and one has the Captain. Go into the one where the Captain is. There will most likely be a few elites guarding the cells. Take them out and open the cells with the switch at the other end of the room. Once you have acquired the Captain, proceed through the halls back to the large three level room.
Be sure to watch out for the Captain, because if he dies you will have to restart. Once in the large room, hurry and press the switch just outside the door. After you have pressed it, you should get aboard a covenant vessel and fly out of there(you dont do this). Now you have completed the level! Next up Silent Cartographer.