Your sweepers have priority which is useful, but IMO you lack a truly fast powerhouse that can take down two or three on its own. Charizard is strong but there are other Fire-type options that work just as well (e.g. Blaziken, Infernape, hell even Delphox)
Also one thing you could try is to not use standard move sets. Leaders do their research (I hope) and they know what to expect from which Pokemon. Doing something outrageous like sticking Iron Head on a Garchomp could land you a free kill when they least expect it. I've seen (not recommended) people use Lick on Greninja just to use it as a makeshift spinblocker.