About temptation, I think it's not only a magical attack. Even without magic, the body of a succubus is a weapon who inflict a huge temptation to anyone. Iron will , in my opinion, should be a magical help to fight against the magical seduction but with the normal seduction too who is passively affecting you just by watching your opponent.
You don't like the gauge, so we can have the duration of iron will reducing if the succubi use a temptation move. Maybe Iron will will have 8 turns, but it loose 3 turns each time a temptation move is used. And if we aren't protected, we lose HP every turn because our mind isn't protected.
And some attack can affect Iron will too. For example the whip, it is a sexual tool and a weapon, so it make dmg to our HP, but it can dmg the duration of Iron Will.