After unpetrifying Rosabeth of Melfia, speak to him and exhaust all his dialogue, he will now be able to be summoned for 3 boss fights, you will need to summon him for at least 2 of the fights and keep him alive. These are the boss fights and his summon locations:
*** Spoiler - click to reveal ***Prowling Magus and Congregation -- Brightstone Cove Tseldora -- Inside one of the tents near the bonfire
Looking Glass Knight -- Drangleic Castle -- Left of the boss fog
Giant Lord -- Memory of Jeigh -- Bottom of the stairs near the blocked exit, before the fog
Once you complete three of these bosses with him alive, find him at Memory of Orro. Exhaust all his dialogue and he will give you his weapon and equipment. If you are unable to keep him alive through three different bosses, you can also use a Bonfire Ascetic to fight a boss again, this will count towards the 3 required bosses.