At the very end of Township you will find Bishop Three inside a wrecked and rusted ship in the relative middle of a large open area.
You will know you are about to come across this area when you enter a large hangar with an unmanned tank right in your face. There are also two enemy tanks that will spawn right after you exit this hangar so I would highly advise using this unmanned tank to destroy the other two.
After you arrive to where Bishop Three is laying you must simply defend yourself (and him) until help arrives. At the end of the level is when your achievement should pop.