The card reads when damage is dealt to it, reduce by 1. Is that damage from a single spell or single player? That is, if a player casts two spells that cause two damage each, is the correct math (2-1)*2=2 or (2*2)-1=3?
Cauterizer has "When damage is dealt to this minion, reduce that damage to 1." on it. Each separate spell or ability is a new instance of damage. So if you cast 2 spells that each deal 3 damage, Cauterizer would only take 1 damage from each for a total of 2 damage.
Hope that clarifies it.