[Brandy]

Is this how this would work?

First, the 20 points of damage is reduced to 2 (Basic, p. 380, Damage to ...Diffuse Targets).
Then, is this a situation where you would invoke the text from p. 378, "...if the subject has no ST at all or is not resisting, use HP instead"? I think I would, and the most likely outcome, assuming that the Diffuse Creature has at least 4 HP is that the attack would do no knockback.