How do I make a mob take more damage from a specific item

It's actually very simple. Go to the sword you wanna add this into, then create a new "when living entity is hit with tool" trigger, then add "deal ... damage to Event:Target entity

You can select the damage type. I recommand you put "player" in but you can make it so that it's fire damage for example or magic so that it bypasses armor.

But this will make all mobs take more damage! @tdo

You can make an "if procedure" with a "is [entity] subtype of [choose the mob you want]" (you can find this at the bottom of the "LOGIC" tab) to select which mob you want the item to do more damage too :)