My best method is to stay far away and wait for him to run at me while his Chikage is non-transformed, then ripost him the moment he's about to stab, preferably with a pistol as opposed to the Blunderbuss or Ludwig's Rifle. Works 95% of the time, offering an easy visceral, followed up by a long enough window for a charge attack, but it might take some time figuring out the right time to fire.
Get away another 3-5 backsteps, avoiding his quickstep-southpaw patterns and numbing mists, rinse, and repeat. He may or may not use a blood vial at some point, but you probably won't need more than 20 bullets to finish him off. When he transforms his weapon, he does become more predictable, but don't get too ****. Let his health drain on its own, if you must. Above all, don't forget to equip gear with great Blood DEF, like the Cainhurst Armor or the Knight's Garb, or better yet Maria's Hunter Garb if you have it. These sets also have pretty good Thrust resistance. Most often, you'll probably be dying to his Repeating Pistol, though, because of course he has unlimited bullets.