I don't see it all that hard to code.
A thrown grenade could have an immediate invisible area of effect thing that the AI can interact with. That would make the AI avoid areas where grenades are thrown.
You could even code some enemies to not avoid this particular effect based on their intelligence. When fighting the undead, or beasts they would not avoid the area of a thrown grenade, while intelligent humanoids might understand what a grenade it and try to avoid the impact zone.
