If the PC is actively resisting the spell somehow; concentration, counterspell, moving around to not be an easy target, distractions, etc. Why not award 50%-75% the caster would receive for successfully casting the spell?
I award for resisting spells, but using an entirely Elemental Companion - Magic based world, most spell resists consist of "get the heck out of the way of that <insert element name here> bolt coming at you!!!"