Heck, I can even say that a standard-issued Mach Strike deck can hold against a Expanded deck. The Garchomp is capable of dishing out 200 damage in a single blow. That’s enough to take out most EX/GX units, never mind the regular cards.
There is more to being a competitive deck than dishing out big damage. Against an Expanded Deck that is at least competently built and piloted, Mach Strike only wins via luck.
Mach Strike is still one of the strongest Theme Decks in the Theme Format, but even as its lead erodes it remains popular because it tends to either set up quickly (and win) or lose quickly, in an obvious fashion so that insta-quitters don't lose a lot of time figuring it out. Mach Strike wasn't even leaps and bounds beyond the previous two Theme Decks; in a sense, it got lucky that TPC doesn't really care about the Theme Format and that it released at a time when it utterly replaced its immediate predecessor (Clanging Thunder) while lacking a true natural counter.
What is a natural counter, and why does it matter? Usually, it is just a solid deck that exploits the Weakness on your deck's main attacker, but sometimes it is strategic. Garchomp is [Y] Weak, with a decent amount of variety in the Weaknesses of the deck's secondary attackers (Gumshoos, Lucario, and Hippowdon); we just don't have a good [Y] focused Theme Deck and it looks like we aren't getting one either. This is a place where the devs might be able to step in. It doesn't take a "broken" [Y] deck to make a difference; the top Theme Deck in the Theme Format metagame can be taken down a notch so long as one (or one more) poor matchup exists for it.
The other form of natural counter is harder to build, but we have an example that also explains why Mach Strike has remained good for so long; it counters Tropical Takedown because Alolan Raticate and (more importantly) Exeggcute are small enough for Quick Dive to OHKO on the Bench and because the Tropical Takedown deck seems to be built assuming Alolan Exeggutor can take at least one hit. Even without Type advantage, this means a Tropical Takedown needs a lucky, fast open (or for the Mach Strike deck to misfire) to really win.
The final thought I want to leave you all with is that, if I were to assign traditional, western (American?) letter grades to our Theme Decks, Mach Strike is at best a C+ because I am not grading on a curve. Theme Decks need to teach a new player how the game works, be a solid starting point for said new player's own first deck, and have a little something to it for experienced players (even if only in the Theme Format). Some of the Online Exclusive Decks have gotten it right (or more right) than Mach Strike... but that meant they dominated the Theme Format so severely said decks had to be nerfed. =/ Most Theme Decks have been D's or F's; a lot just seemed better because when you've got a lot of "low F" decks, a "high F" seems average and a "D" seems good. XP
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