Wallriding is when you can exploit certain vehicles to temporarily ride on walls. You can do this on multiple levels and this tactic is great for finding sniper positions.
On this level, take a Sniper rifle and a Spectre and boost on the wall near the rocks and the spotting area for the Carbine and Battle rifle. It will take you a few tries. By holding A, you can do a wheelie, which will allow you to have the bottom of the Spectre on the wall. Speed boost and wheelie up the wall (it may take you a few times to perfect the maneuver). Once you are on the side keep on boosting and go left, which will take you to the unreachable area, a perfect sniping spot.
You can wall ride on Coagulation. It will work at either the red or blue bases. At red base, get a Spectre or a Ghost, and go by the rocks behind the base. By holding A, you can do a wheelie, which will allow you to have the bottom of the Spectre on the wall. Speed boost and wheelie up the wall (it may take you a few times to find the spot, but it is near the rock closest to the base). Once you are up, keep speed boosting and you will end up on a slightly slanted wall. Get out of the vehicle and you can stay on the ledge, but the Ghost or Spectre might fall off. You can do the same at blue base, and you can do the same near blue base where the high perch/ledge. Just speed boost and wheelie up and your set to snipe people. You may want a rocket launcher, just in case a vehicle comes up to you. Also, since you are so high at the red base sniping spot, you cannot snipe people farther than the middle of Coagulation. This spot is near the rocket spawn.
Note: there is also a place in Containment at the back of red base where you can wallride with a Spectre.
you can also do this on The Ark on Halo 3.