http://en.wikipedia.org/wiki/Operation_Market_Garden
http://en.wikipedia.org/wiki/Battle_of_Monte_Cassino
These are but a few examples of superior German leadership, training, weapons and infantry turning the tide of battle, and defeating much larger Allied forces that also had air superiority. A user(Affe I think? I'm not quite sure on the spelling) had quite a detailed list of battles post-Battle of Stalingrad where the Germans slaughtered Americans and Russians in great numbers, and even launched counter offensives despite having no air support and were heavily outnumbered.
To answer your question of "where did these battle hardened troops come from?", my answer is troops who fought in the early war(Battle of Poland, Battle of Denmark, Battle of France and etc) as well as in Russia. A lot of these troops had more experience than any other nation in the war except maybe for the Japanese, and so they carried their experience and skills over into the later stages of the war; the Obersoldaten are a direct homage to those very troops who survived the countless battles from the late 1930's until late 44'.
The CoH franchise prides itself on its stellar gameplay mechanics and historical authenticity, which is why nerfing the Obersoldaten to make them like any other infantry is an error of the highest magnitude. The Obersoldaten are supposed to be the best infantry in the game, and even now Shocks and conscripts/US rifles can defeat Obersoldatens if they're supported with light artillery(Mortars and etc) and light vehicles. The Germans were the most elite in WW2 so obviously the Obersoldaten should reflect that since CoH2 is supposed to be historically authentic.
One of your examples of superior german leadership shows a german defeat, and you keep showing operation market garden like it was the most crushing defeat the allies had. Yes, even in 1944 germans could still win battles, not denying that.
Still no evidence of that "slaughter" of russian and american forces.
Also lol at japanese being well trained troops, what are you even smoking?
Yeah, sure the germans launched counterattacks with very little air support and whatever, and you know what ? They all failed miserably.
The troops that survived from the start of the war to 1944-45 barely existed. Sure there were a few, but these kind of people very rare due to the nature of infantry combat.
And there is absolutely no evidence that more germans had more of these troops than anyone else, considering that their divisions were constantly getting annihilated and reffited over and over again.