From Halopedia, the Halo wiki
Science Mountain Walkthrough
The level Science Mountain is not that complicated. You start off with 2 Scorpions, and a marine. You have to protect the pelican at the start, by killing the ground troops and phantom. Then, you have to get to the clearing, and clear the new enemies. Afterwards, you will get another Scorpion, and you'll have to destroy the shields to continue. As you continue through the path, you'll get ambushed. Fend off the enemies and you'll have to advance on foot down a sidepath, as a shield blocks the way. Once you get to the end of the sidepath(on the other side of the shield), you'll have to clear the area to get another scorpion & a marine. Once you get passed the shield, you'll go through a small cave, with a few enemies. Be careful, though, as you can end up dealing damage to yourself. On the other side of the cave, you'll be able to turn left or right. Turn left for a spare scorpion(I recommend you save it for later), or turn right to face a clearing where waves of enemies will attack you. Be careful of the knight commanders, as they have super-destructive Incineration Cannons. After clearing all the enemies that keep on spawning, continue down left and activate the terminal. You have finished the level.
There is a way to skip most of the footpath area, and overpower it with a scorpion. You can drive through a dip in the wall, if you use a tank as a bridge. Use the video link below to see how.