I found strategy guide here that states:
Killing supporting unit Imagine you want to kill a special unit (say, a hero) but don’t have the force to kill all units in his area. Let’s imagine you want to kill the Gantrithor, supported by a Zealot. Now, your enemy will be confident since the Gantrithor can’t be splashed by simply killing off the Zealot, a tactic you would normally use. But there is another way: simply set a air-unit like the Valkyrie against the Zealot. It shouldn’t be hard to penetrate its health, and then the supporting unit will be destroyed. The only defense against such things except of course to avoid these pairings in your areas is to build the Defense Module.
I understand that the Gantrithor should be immune to ground splash damage since the Zealot is a ground unit.
However the Valkyrie can only target air so it can't target the Zealot, did I miss a rule somewhere? Can you attack a support like this? Or is it just an air splash damage that triggers? You normally need to kill the front line unit for a splash to happen, if this was a Wraith I could see this working, but can someone help?
Edit:(I did miss a rule) The strategy is done WITHOUT using splash damage at all, see bolded part in the answer.