The potential for Odin to do something crazy with the Tesseract or Infinity Gauntlet is the only reason anyone should think the Asgardians stand a chance, because man-to-man they're laughably outclassed. Mjolnir!Thor is a dramatic outlier, as are probably Odin and Heimdall. The Destroyer might be a threat, sure. Loki is not a lesser Asgardian, he's a small and unusually talented frost giant, so his evident ability to take a beating says nothing about anyone else in their ranks except, by extension, Thor. The Warriors Three and Sif are clearly supposed to be extraordinary even for Asgardians.
Even if we (perhaps favorably) give a good whack from Mjolnir or blast from the Destroyer the kill on any given Kryptonian, the bulk of the Asgardian force is still set for a wrecking. Superspeed and cover from two guys with heat-vision are probably good enough checks on the Casket of Ancient Winters. I don't need to think they're all as capable as Faora, Zod or Nam-Ek to assume the sun-boost would be commensurate, and between the three of them they've tanked missiles, gone ground-to-air on fighter jets even without flight, hurled trucks and train cars, survived exposure in space and atmospheric reentry, been punched across a cityscape without serious harm, and literally demolished skyscrapers. Even muted versions of what they can do outclass all but the best Asgardian by magnitudes, from what we can infer.
Then there's the World Engine, and the gunships the Kryptonians brought with them. The bulk of the Asgardian army could easily die by that artillery alone after being surrounded and hemmed in by the faster aliens. Even if Thor takes it all out with the storms and a Jotunheim-style bringing down of the hammer, his folks just aren't tough enough. Without him they'll be quickly overwhelmed and killed; Faora and Nam-Ek will handle Sif and the Warriors Three, Kal-El will just slap the Destroyer into the Long Island Sound and join Zod in beating down Odin while some spare Kryptonians take on Heimdall, Loki gets chased around by some more spare Kryptonians, and then Thor's all alone.