Element boosts stat?

What I would like to happen is that if an elemental skill is used against the 'wrong' sort of enemy, it does very little damage and also changes a paramenter.  The "very little damage" bit is easy with the element rate in the enemy's Features section.  It's the other bit on which I'm stuck.

For example, using a fire spell against a fire enemy boosts DEF.

I began by thinking that in the troop event I would have an event run at turn 0 which inflicted a state on a particular enemy.  Then maybe there needs to be some way of the damage formula of the fire skill checking to see if that state is present.  If it is, then buff the enemy's DEF.  However, I have got myself in a tangle with the formula.

Can anyone help me out?  Or suggest a more efficient/effective way of achieving this?

Thanks.

My idea is that you create a new State and put no information in it, except maybe the duration set that to 1 turn



Then you put that Fake state on all your elemental skills involved with fire



Lastly in every troop with a Fire Element Enemy you Do a check every turn for this Fake state on only the fire Element Enemies then apply another state to them that would give them the stat boost

Thanks for the suggestion.  I'll try that out when I get a chance (RL a bit demanding at the moment).