Potency helps determine the chance of landing a debuff. If you are using Phoenix, counter-attack chance should be high if you have Kanan's unique leveled high enough. Potency can still be good for landing stun on him though if Zeb hits him while he already has Daze, Stagger, or Expose.
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Edit: you need to post a swgoh.gg account. People can't advise on mods without knowing what you have.