How to beat Mace when CG refuses to fix hound tenacity check.

You do, but without knowing which ship the OP is using to try to debuff, cannot do that side of the math. Also, not sure if the bonus tenacity is applied after potency is taken into consideration or not, but lean towards the bonus applies after tenacity - potency. As tenacity cannot be reduced beyond 15%, then adding the 50% would still give a 65% tenacity in round one. It can also be affected by the other ships in the squad, such as ghost (-25% potency to all enemies).