As far as I know, there is no way to set a certain target for the AI that they prefer on auto, so a real solution for your problem doesn't exist. 

You could do the fights manually, or you could, while the fight runs on auto, semi-manual chose the target by clicking on it, that will lead to a red circle around the chosen target and all of your champs will attack it.

Or use champs with AoE attacks and chose these attacks as prefered skills in AI setting.

AI priority is a bit finicky, but broadly speaking,

• AI will typically prioritize affinity over most other things. That means if there is a strong-affinity target, it'll usually be chosen, and if there's weak-affinity targets, they'll usually be avoided.
• AI will also typically prioritize killing targets it "thinks" it can kill. How it concludes that is a bit opaque, but usually you can judge it by the enemy HP being <= 15%.
• If neither of the above factor in, then the next priority is generally debuffing. It'll also usually prioritize crowd control debuffs over other types. This is important because, for example, if you were running Royal Guard, it means that the AI will sometimes choose to use the A1 over the A2, because the A1 places a debuff while the A2 does not.
• If none of the above are relevant, then the AI will generally choose the most damaging ability it has, and target the lowest-remaining-HP target.