As you know, magical shielded units are imune to most spells (except angel of deaths, swamps, hypnotise, land modifier spells and lava from spells.

 

But you can still strike them with lightning or swarms. All you have to do is cast the spell not directly on the shielded units, so it won't get reflected. 

 

You can cast some direct spells like swamp or hypnotyse.

 

Watch this small video: