How to make a mob attack mobs under a tag?

I have been trying to make a mob that attacks either a group of mobs with the same NBT tag or something else, but I can't figure out how to use the attack conditions (I have used spawning conditions before but I don't understand Additional Start Conditions), can someone please explain how I to get the target mob's (the one being targeted by the other mob) NBT Value?

I know I can just make a lot of Attack Tasks in the mob's AI but what do I do when there's just a lot of them? 

If someone can explain this I will be very thankfull.