Contact damage during invulnerability
What changes
Outside invulnerability these wisps do not deal contact damage. During it, their contact damage is twice Isaac's damage minus five, with no positive damage at 2.5 damage or below.
How to approach the setup
Keep Unicorn Stump combined with the book. Activate it before moving the wisps into an enemy, then retreat before the short invulnerability window ends.
A practical suggestion for this pair. Check the rest of your inventory and the room before relying on it.
The caution to remember
Unicorn Stump prevents Isaac from firing tears during its effect. A weak damage stat can leave these wisps with no useful contact damage; their presence alone does not make ramming an enemy productive.
What this teaches about a run
Protection and damage are separate conditions. Check that the attack works during the protection window before committing to close contact.