To unlock this one you need to have both Phiera Der Tuphello and Eight The Sparrow equipped and level 8. You also need to have the Tiragisú passive item equipped (this does not need to be levelled up). Tiragisú is unlocked by surviving for 20 minutes as Krochi Freetto. Once you've met these requirements find a treasure chest and open it to evolve the weapons into the Phieraggi.
The easiest way to do this is to use the Pugnala Provola character who is unlocked by opening the coffin in the Mad Woods. He starts with the two weapons so you'll have a head start.