I disagree with that wholeheartedly. The Defiant IS a destroyer.
Though two of them and one Akira seemed to be sufficient to try and retake the Prometheus from a pair of Warbirds (admittedly, these could just have been scrambled like the Nebula that was destroyed in the same episode was, and just had the luck to arrive together) so they must be punching far above their weight.
The U.S.S. Defiant, however, is just a ship with god syndrome and clearly more powerful than it should be.
The phrase you're looking for is 'Hero Ship' which applies to a couple of individual ships throughout the series (the Enterprise being the main one), so I wouldn't single out the U.S.S Defiant as a lone offender in this case :P
I'm not going to go through the numerous blunders that they give the Defiant because the list is too long, but I'll start at how the Odyssey gets no shields against Dom figthers in Jem'Hadar, but the Defiant never has that problem (where the rest of the Feds don't adapt shielding until "the ship" is recovered).
Well, in the first encounter with the Dominion, the Defiant doesn't fare much better than the Odyssey did (it did get disabled and boarded). It was also noted to have ablative armour, which seems to be a feature unique to the Defiant class.
Defiant class ships are powerful for their size, more powerful than a bird of prey, but not as powerful as capital ships. It would take a very skilled crew and a very incompetant crew for a Defiant class to defeat a Galaxy, Nebula (which were both upgraded for the war), Akira, Prometheus, or Sovereign Class vessel.
Well you do see the Defiant go head to head with an upgrade Excelsior class (the Lakota) and the ships were roughly equal, though both were somewhat holding back (i.e. not using torpedoes), so it's not like the Defiant was written to be the 'be all and end all' of combat ships.
The Sovereign class is actually Starfleet's "battleship"
True.
as well as the Galaxy class when it had war upgrades.
Perhaps not so true. While I can imagine that the Galaxy class were given additional boosts to their defensive systems, they're never classed as battleships. Strapping missile launchers to a container ship of today wouldn't mean that you now have a cruiser
[/quote]I'm just not see a little ship beat these ships when they have warp cores larger than the Defiant itself, type XII phaser arrays, and multiple photon / quantum launchers.[/quote]
Maybe they could, maybe they couldn't. Don't judge a ship solely by it's size. The synergy of it's design matters much more for combat that pure statistics. And besides, we may get the impression the Defiant is a god ship, but there are many occasions where it's equalled, or outgunned by enemy foes and has to use the competence of it's crew to win the day.
The Akira has FIFTEEN photon launchers!!!
It was also meant to be a fighter carrier, but that ability wasn't shown on screen either, so you have to take any background information like that with a grain of salt.