To get the Excalibastard you must have a badass rank of at least 2500. This is achieved by doing the various challenges in the badass rank section of the
menu. Once you have 2500 travel to Stanton's Liver. There is not a fast travel machine there so you will have to go to Triton Flats first and enter Stanton's Liver. Once in Stanton's Liver walk forward until you see some enemies and a bridge. Take a left before crossing the bridge and you should notice a path that follows the wall. Continue on it until the end to find the gun. If you are having trouble finding anything I added a few pictures to help.
Stanton's Liver Map -
Triton Flats Map -