Alright. First you gotta go to Blood Gulch with ghosts turned on. Then go in front of the red base a little with a ghost on the first hill. Aim towards the blue base. Start starfing right without changing where you are looking. If you get out just before you hit the wall at the slanted part get out and the ghost should push you up the wall. You will then be on top of the level. You will know you are up when you get the because you will be standing on nothing but still be able to see the level. It helps to bring a sniper. Watch out though cuz your friends can still see and shout you if you don't hide behind the peaks and rocks.

You can bring a ghost up there too. You can blow it up there with a partner using rockets and grenades or you could do it the easy way. First put a ghost up the wall where I told you you could get up before. Have it all the way on the wall facing the ground. Then get a different ghost and be far away and then start floorin it towards the ghost on the wall. Start holding A to lift up the front end. If you get it right you should start "driving" up the wall. Keep pressing foward until you reach the top of the level.