How to make macro to target mobs that are aggrod and not slowed?

I'm playing mage and what I'm looking to accomplish here is to spread my Frostbolt's slow effect only on those hostile NPCs that have been already aggrod AND which aren't under my Frostbolt's slow effect right now, already.

This is for dungeon runs, where I do not wish to pull any mobs the tank hasn't already pulled.

So I want a macro that targets a random hostile NPC from among hostile NPCs that are currently attacking someone AND which aren't affected by my Frostbolt currently.

How do I do this?

Macros aren't that advanced.