Mission 2:
So there is one part where you have a warthog and you can even pick up some ghosts, however at the end of the canyon there is a lip on the ground preventing you from going any further.
There is one of those floating towers right next to it. Blow it up, then melee a piece of the tower next to the lip, and you can use this to get your Ghost over. There is a narrow passage way shortly after which only the ghost can get through (At least with single player), so don’t bother with the Warthog
Don’t know if I’m the first to do this, so I won’t bother with credit. I hope somebody finds this as useful as I did.
Peace