brass pipe thread loosening
An old, also works, dumb sounding way, to loosen threads in any materials is to tap(sorta beat) within reason so you dont flatten or otherwise ruin. Copper and brass are more prone to distortion than say galvanized pipe. But IF you cant quite get there with instrument of choice, use a punch type device. The 'best' tools for such operations are two hammers timed to hit opposite of each other at exact same time.AND tapping equally all around the circumferance. It isnt rocket science but comes easier with a touch of practice. Tap a little, try it, tap a bit more, try more. IF it is hex headed use a hex wrench or socket. Not a 12 corner!! A hex head puts more even pressure around the circumferance!
I little more info as to what and where this is may be more helpful!!! And size??? Sometimes just a little longer cheater bar is proper tool!!