Partway through the main story missions you’ll come across a few side missions about Tombstone and his biker gang. Eventually the last mission in the chain will have you confront and fight Tombstone at his headquarters- just defeat him to get the trophy.
As a boss he’s not too bad. Perfect dodge his attacks to get some time with him and land a few punches, then retreat and repeat. His main attacks are pretty powerful if they land, so just keep dodging until you get a perfect dodge that slows down time for a little while.