This is a well thought out magic system.

So like fighting a robot (Magitek) covered in in rubber as an example: Lightning will not essentially hurt it at the moment because of the fight. So you would combo one of the following?

Fire starts making the rubber melt or loose. After casting fire then combo shows up to any of the other elements. Fort this example let's use Ice/Lightning. Ice cause the rubber to harden and makes it shatter from rapid cooling, or the fire previously melts the rubber. Now the robots circuits are exposed and lightning will now do double the damage.

Makes it sound more fun than spamming thunder the whole time.