It's been awhile since I've played.

I have everyone prioritize killing mages first, then archers (especially in Awakenings). Then they target whatever Alistair is targeting.

Alistair uses taunt anytime someone else is getting attacked. Warcry whenever it's up.

I think I always manually controlled stuff like sleep/horror. Generally that works best for the AOE stuff. I usually avoid most of the AOE spells if they affect friendly targets. Or at least, I don't let the AI control them, because it's idiotic about how it does them.

I think the tactics are done in order so if it's earlier in the list it gets priority.