I am not sure about how this AI works 

but I think it will be like this

1st is Attacking the weaker elemental hero. If there are more similar element, attack the one who has less HP left.

So, they don't just randomly attack if more than one weaker element in a battle.

2nd is Attacking the one who can be oneshot regardless the element (same or weaker). (Probably the lowest health at the beginning of a turn). But I assumed it won't attack stronger element. 

3rd is Attacking the one who is not invincible. But sometimes, they will still attack the invincible with critical condition even though there is another who is not invincible.

That happens occasionally when Krau wants to oneshot invincible C.armin but can't... Dunno why he chose him rather than my Cermia who was not invincible. (But that is back to 2nd step)