Don't get hit.
Also, Zamus is pretty light, so G&W's hard hitting smashes should finish her off as soon as she gets into kill percents.
Until then, use lots of bair, nair, and air combos, and just remember to take her out as soon as possible with full-blown kill moves so she can't recover. Zamus has ridiculous recovery if you don't edgehog her, and might bring you down with her if you do.
She's also useless when she's above someone, but her dair will get her down quick if you don't knock her out of it with G&W's insane uair, nair, usmash, utilt, etc.
I'd also recommend the Summit or Frigate Orpheon due to the lack of edges for Zamus to try to recover. Makes it twice as easy to edgehog, cuz you don't need to. : D