Tenacity down and potency in the heroic pit raid

I think your missing the level difference in the rancor raid. They are 5-10 lvls above your toons. Even with tenacity down. They will still resist more because of this level spread.

I think I saw somewhere that resist chance is something like:

(Tenacity + 5*level difference - Potency)

With minimum resist chance of 15%.

So even if Tenacity was 0, you still need 100 potency to have a descent success rate.