I am trying to create a spell that, when cast on an enemy unit, increases the amount of damage that the target unit takes when hit with a spell (notably, it is a "weakness to fire" spell). Is there a trigger action that simply deals X damage to a target?
Unit Life, its in the unit section, depending on how you plan to use it exactly there are a few such as Unit Life, Unit Life (Percent) Unit max life ect.
If you want the unit to become weak against all spells, just give it a modified spell-damage-reduction ability that has a negative damage reduction (so, in effect, the damage is increased)
Hmm... decrease damage only to fire? That sounds damn difficult and I can't really find a way for it. Hmm... perhaps take all fire spells and do a trigger... I will try to make a reflect spell so... if I succeed I'll tell you.