First you must go to the landing with the fountains on it.
Just as you go up the stairs (towards the broken ledge where you can take cover) turn around.
There is a flat looking podium-shaped thing to the left of the stairs.
Move to the right side of the stairs and line up your head on the podium.
Press A, B and Up once with the lancer and you will slide over to the podium.
You're now floating over it so you press B and you will be on top of the podium.
You can now walk on the fence out of the map by the tree, or follow the fence around by the lower landing's spawn.
You may be able to fall on a small ledge by the bottom if you try to roll off and keep going around.
I only did the last mentioned part once and wasn't able to go around completely since time ran out.
Have fun and show your friends!