As i understand it, it isnt the lowest hp that takes the stun it is 

1) neg affinity

2) lowest percentage of hp left.

So for example, my desired stun target in my unk has the highest hp, but lowest defence to ensure she gets hit hardest and has the lowest percentage of hp left.

What is the defence and hp (full) of both the champs? in case i am not understanding?

Geo 28k hp and 1.6k defense
Heiress 31k hp and 2.1k defense

They are both force affinity champions so weak vs spirit.

When the stun goes off they are full hp with block damage buff as well.

stun target needs the highest hp and lowest defence 

Geo's defense is too high still relative to Heiress' - should be closer to 1k, max 1.2k (and yes, that requires actual effort to get the defense as low as possible, it tends to make the champ in question rather hard to use elsewhere). 

Also, do you have any healing in this comp? As Trevor mentioned, in the classic Maneater/Painkeeper unkillable, you want the stun target to have very low def but at the same time the highest max HP of everybody - because that way, when PK does her heal based on her own HP, it will heal a lower percentage of the stun target's total health than any other champion's, making sure that they are left with the lowest percentage HP at the time of the stun. Not sure if that same rule applies here, because if you're not healing anyway, or doing heals based on the target's HP rather than the healer's, that part would be different. 

Ya between demytha heals+deacon leech, and being block damage for every cb turn everyone sits at 100%hp the entire fight.

I will try to get geos def lower, it actually gets hard trying to keep stats so low :/

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Thanks @L9753 & @Trevor Wilson

The hp and def ratios were indeed too close. My pockets are empty of silver so this gimp geo was cheap solution for the moment.

Back to silver farming to get a more permanent solution set up.

 Ah, someday... I have everybody but Seeker and Deacon. My stockpile of ancient shards is building back up rather quickly, with a little luck I'll pull those two champions during the next fusion. :-)

Glad you worked it out.

In my team there is a massive difference in defence for my stun target but still- maybe one every 20 runs there is a freaky set of crits and the stun goes wrong btw.