Stick a Flea Man in a giant steam powered mech & battle on an outer wall of the Castle, perhaps a courtyard. Or maybe like 3 in a few large-ish sized mechs & fight 'em all at once. You're welcome, Konami.
There's no apparent way to confirm this, but I think that the second boss in Castlevania: The Adventure ReBirth "is" a boss version of a fleaman. Or at least Igor from the original Castlevania, who gets beefy for a time like Mr. Hyde, until it wears off. Plus the "executioner" from Mirror of Fate might be a boss version of a hunchback. But yeah, if they'd put a flea man in a suit of armor with insane axe attacks - a flea man in a steam powered mech would be very fitting!