To unlock this one you need to equip the Vento Sacro and Whip weapons and the Hollow Heart passive. Level both weapons to 8 and evolve the Whip into Bloody Tear by opening a chest. Then open another chest and the Vento Sacro and Bloody Tear will unite into Fuwalafuwaloo.
This is probably easiest to do with either Zi'Assunta Belpaese who starts with Vento Sacro or Antonio Belpaese who starts with Whip. You could also use the Mad Forest map which starts with a Hollow Heart to the north that can be picked up. Either way, both characters will require two items that they don't start with.