You need a friend to help you with this one. It is very difficult to do and takes lots of time. First of all, go to Blood Glutch, and have tanks, warthogs, and ghosts. Drive into a tree, and have your friend get in the warthog and push you against it. After about 500 tries, your tank will take the form of the tree and then become immortal and have consecutive ammo. This works online, too, and with rocks.
Submitted by: Thor