To unlock this one you need to equip Laurel, Metaglio Left and Metaglio Right. Level them all up to max and then evolve them by opening a treasure chest.
The Metaglio Left buffs you whilst the Metaglio Right buffs your enemies so I'd level that up after getting a decent build going.
The Metaglio items become available after unlocking the Yellow Sign:
I did this in the Mad Forest map. The Metaglio Left is at the left of the map and the Metaglio Right at the right so you'll have some running to do (the Wings passive item may be handy for you). Each Metaglio item is guarded by a boss so make sure you're ok to kite it around whilst killing it.