This achievement is the union of "Vento Sacro" and "Bloody Tear".
You will need to unlock "Vento Sacro" if you haven't already, it is unlocked by surviving 15 minutes with Zi'Assunta. (Achievement "Vento Sacro - Survive 15 minutes with Zi'Assunta."
To unlock the Fuwalafuwaloo achievement there is two ways:
You can start with the character "Minnah Mannarah" whose starting weapon is "Bloody Tear" and then you will just need to get "Vento Sacro".
Vento Sacro must be level 8 before Fuwalafuwaloo is available. Once it is at level 8 the next chest you pick up will give you Fuwalafuwaloo.
Or
With any character you can Unite "Whip" + "Hollow Heart" = "Bloody Tear"
And then Unite "Bloody Tear" + "Vento Sacro"
The achievement will be rewarded after the end of the run.