This is the only border you can get ontop of without a friend. Go to the Sidewinder level. Go ontop of one of the ledges on the inside. You'll see some ledges on the tunnel support near the tunnel that leads between the two bases. Throw a grenade, then about a second later, jump and shoot a rocket down. If you timed it right, you should fly up. Wait for your shield to regenerate, then throw another grenade and get up ontop of the level.
Submitted by: Thor