Also Spartans-II's are augmented super soldiers trained from childhood in order to be the ultimate solider so the feats they pull of are justified.
Not really (IMO). Four soldiers should not be able to consistently win against 10,000 to 1 odds, especially against an alien society that has more advanced technology than they do (or at least on par equipment), and are an organized militarily in their own right. Plus with the Spartan III's and IV's any Joe Schmo can become a god-like 1 man army (case in point Sergeant Buck).
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In the Forerunner Saga, the Forerunners were at war with a very advanced human society. They beat the humans almost to extinction but then, the humans were fighting a two front war. One of them was the Flood.... and they conquered the Flood or at least pushed it back way out into the fringes.
All that is doing is pointing to how big Mary Sues the Halo humans are.
We were sailing between the stars long before any of the Covenant races even lifted their faces out of the dirt. "Ha Ha Primitives!" We were also so awesome that we were winning a war between us and species that makes the Reapers look like tinker toys in comparison, and the only reason we lost is that we were fighting a foe on two fronts. Even then, we survived the massive galaxy wide purge, protected by the very species we were just kicking the crap out of (because we're that awesome). A species that literally let itself go extinct while keeping us safe. And then, in the modern timeline, we not only beat back the combined species of the Covenant, but the Flood as well. Every facet of the Halo-verse is saturated with humanity being front and center.
At least with Mass Effect there are some aliens present that we don't immediately shoot in the face and we allow to tag along with us.