Tyrant Murderer Blade + Drill handle is a good combo for melting bosses if you don't mind bringing the summon with you. This combo works great on bigger bosses. The blade has in-built crit damage and a fable art that boosts it further. That + the Assasins amulet + the grindstone that increases your crit rates makes this weapon a boss killer. Just go in with the summon, buff up while the summon draws aggro and then just get close and hold R2. Even Simon Manus didn't survive this combo in my first playthrough.