The main problem with what you are trying to do is the fact that you cannot change the values of an actors element rate without external scripts.
and simple enough to read the elemental rate: http://i.imgur.com/E8uCWL3.png
But I have spent another four hours today trying to find a way to overwrite the element rate, eg:http://i.imgur.com/pDSZO4V.png
The closest I've come is the "States" whith which I could make 40 variations of and add/remove in order to form a sliding scale of 5%, from 0 to 200%.
Is there another way I can overwrite the FEATURE_ELEMENT_RATE?
Once I can overcome this milestone I already have a ice skill to do "a.atk * a.element_rate(3)" damage. My plan is then to run a 'Common Event' that checks if the opponent was defeated, then run the above script to improve ice affinity and remove some fire affinity.