I'd have to agree with Shor, considering throughout TES history it has been the the human empires that have been running the show.
Also the argonians didn't just hold back the forces of oblivion they charged into the gates themselves in such numbers that the Dremora commanders were forced to remove the sigil stones to close the gates.
That's an impossible question to answer. Open battle depends on a lot of factors like tactics, environment, numbers... One on one battles are dependent on opponent physical characteristics. If a Redguard manages to summon a Shehai, he'll be able to destroy entire armies. Powerful Illusion magic can turn enemies against each other. Heavily-armored Orc will struggle in swamps. Lightly-armored Argonians will have hard time in open plains. There are way too many variables to account for.