To be fair, even before Ash screwed up by using Seismic Toss, both BF Charizard and Dusclops seemed roughly comparable. The implication is that BF Charizard was stronger than it, but not by much. Dusclops couldn't be that much weaker than Charizard, otherwise it wouldn't have been so back-and-forth prior to Seismic Toss. Especially since BF Charizard was slightly stronger than Regice, who was slightly stronger than Dusclops.
Also, Dusclops being stronger than BF Charizard wouldn't even make sense from a writing perspective. If it were stronger than Charizard, then why would the writers have Ash make an obvious mistake when they could have had Charizard lose legitmately? The answer being that BF Charizard was slightly stronger than Regice, and also Regice being slightly stronger than Dusclops. The gap between Charizard and Dusclops was simply small enough that Ash's mistake tipped the scales in Brandon's favour, but if that mistake never happened, BF Charizard would have won.
So I'd say it was more like: Regirock > BF Charizard = BF Sceptile => BF Pikachu > Registeel = Regice > Dusclops > Noland's Articuno > Any other Frontier Brain Pokemon. Regice was slightly stronger than Dusclops even despite Regice having basically zero battle experience, and I'd put both it and Dusclops above any of the other Frontier Brain Pokemon. But despite how it looks, the gap between BF Charizard/Sceptile and Dusclops probably wouldn't be that big of a gap.
Remember that Brandon was still the strongest Frontier Brain before he even had Registeel and Regice, and its very likely he was still the strongest Frontier Brain even before getting Regirock. So I think its plausable that Dusclops could have been his former ace, but was 'demoted' to being a non-ace when Brandon got Regirock and it became his strongest Pokemon, and this would be more apparent by the time Brandon got Registeel and Regice. Yet this only goes to show how strong Regice and Registeel are that they, without any battle experience whatsoever, were either equal to Brandon's Dusclops or were just straight-up stronger than it.