May be working as intended, if so I'd suggest amending "... smashing it for 350% damage and knocking it back for 6 tiles, even if it would normally be immune to this."
EDIT: The text quoted is from Massive Blow, not You Shall Be My Weapon, but it ignoes knockback immunity as well, and that may also be okay, so...
See also bug with undiggable walls not giving bonus damage
[1.1.5] You Shall Be My Weapon! ignores knockback immunity
Moderator: Moderator
[1.1.5] You Shall Be My Weapon! ignores knockback immunity
Ghoul never existed, this never happened!