Priority target is weak affin, followed by low hp, followed by leader. The exact threshold at which point it chooses the low hp instead of leader I'm not sure - my guess is something around when CB can determine it might kill the target.

Keep in mind that the low HP criterium is as a percentage of total HP, not the lowest HP in absolute numbers. And of course, this is at the time of the stun, not at the start, so it's not about giving your SC the lowest max HP at the start, it's about ensuring that he has the lowest percentage of his HP left by the time of the stun.  

In my unkillable team, the classic Maneater/Painkeeper one, the stun target (Aothar) actually has the highest HP of everyone on the team, with Painkeeper second highest, but Aothar has massively lower defense than everyone else. As a result, the CB does far more damage on him - then when Painkeeper uses her AOE heal, since that is based on her own HP, it will restore a bigger percentage of the others' HP than of Aothar's (since the others all have lower max HP than PK, while Aothar has higher), leaving Aothar with the lowest percentage HP at the time of the stun, hence he gets targeted. It's pretty reliable, except when the CB crits on Maneater while not critting on Aothar during the AOE hits, which may leave Maneater with a lower percentage of his health and then the run goes south fast. 

thanks for the replies guys, from what u are saying though if they are all on 100% hp and its stunning the person with highest health/def. seems weird! Its not an unkillable team and i cant really control who will be lowest at the time of the stun as i heal through leech. Its a tough one because at one point it was the right stun target then i changed something and it swapped and i havent been able to get it back since!