Buy a stock wrench. It's not the buffer tube that is staked, it's the receiver end plate. The only way to get it off is with brute force, you might chew up the face of the castle nut but they are cheap and it doesn't even matter with the UBR since it comes with it's own hardware and receiver extension.
It sounds like they did not only stake it but loctited it to me. You an use a punch or small chisel or even flat head screw driver and a hammer to carefully knock the staked back or use a dremel and carefully cut the stake. TO break stake rather then constant force or tugging you want to cause i to shear so the wrench hitting it with a hammer to give it that focused energy on the stake. Heating it would help as well.