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From my experience, it wasn't majorly hard if you prepare the right Pokemon. Getting the Silver Symbols is easy, but the hardest part in getting Gold was you can sometimes lose your streak through simple bad luck. The battles against the Frontier Brains weren't actually hard, you just need to make sure your Pokemon are really good for battling. In some sense, fighting against regular Trainers was worse.
The Battle Frontier was one of the most fun things I experienced in the Pokemon series, if not the most. Hope you can collect all of the Symbols too, OP.