Stronger generated flies
What changes
Guppy's Head still supplies two to four blue flies per ordinary activation. Hive Mind improves their damage rather than adding more flies or recharging the active.
How to approach the setup
Use a ready Guppy's Head charge to build up flies before or during combat. Watch your active charge separately from the damage upgrade.
A practical suggestion for this pair. Check the rest of your inventory and the room before relying on it.
The caution to remember
BFFS! is not the blue-fly damage upgrade. A third item can also change the producer: Book of Virtues makes Guppy's Head create wisps that release flies when destroyed.
What this teaches about a run
A producer and a damage modifier solve different problems. More activations need charge support; stronger flies need the appropriate familiar effect.