What zeta should I apply next?

Arena fights aren't long enough for most "stacking" effects to matter much, unless the % stacking is really high or there's a mechanic to swap turns or TM on your team. Rebels don't have that. A well modded Leia triple-tap is much better.

@jawa7778 , exactly what @Vendi1983 said
Oh, and Leia is 1 of the top 3 fastest rebels. She stealths and will probably triple-tap before the other team goes if your CLS, Han, and Old Ben have low enough potencies to get all their debuffs resisted