First you go to the man cannon at Narrows when someone on the other team goes off of it you go too!
Then push the guy that also went off the man cannon and he should fall and die while you will fall too.
But look for a platform under the bridge, jump on that, then you're out of the map!
But watch out for the gaurdians (the machine guns that are automatic), if you jump on top of them too you can get out of the map!