Before you proceed with using Spell Shield, I think many people would agree that there are some problems with using Spell Shield in conjunction with triggered spells. I think its a tricky ability. I will tell you what I mean. If you have a target dummy ability and then a dummy unit that casts another spell on the target, the dummy ability will trigger the Spell Shield and then the spawned dummy will cast another ability that will not trigger the Spell Shield since its already activated. It may also trigger on dummy abilities that are not supposed to be targeted abilities which is not how it is suppose to function. Of course that is not a problem if you dont use any spells triggered in that way or if you dont care about it. You may be able to remedy it in some way but thats to much for me to take into consideration.
However if you still want to know a way to do it I have an idea using a unit group and a special effect array in conjunction with unit indexer. I wont be able to make it now and I may be busy during the weekend. If anyone else has a solution feel free to post it.