I haven't played Bagration in Panzer Corps yet, but I remember well playing it many times in PG1. The standalone scenario, as McGuba points out, is a sure loss, as it was historically. Bagration needs to be that hard, because it's the beginning of the end itself. Before Bragration, Wehrmatch still standed in a solid front against Red Army, and after Bagration... Well, the soviets outrunned themselves and had to halt the offensive due to supply shortages and men exhaustation.
On campaing mode, it's possible to turn the tide, but it still requires a lot of experience. Only way to achieve DV is when you keep a varied and strong core from a long campaign (1941 or 1939 year of start). Overstrengthing and keeping Red Army where they start is key. Eliminate VVS and you will have the edge on the onslaught. Now, if you don't field enough prestige reserves, or you started a 1943 campaing, I'm affraid it's very likely you'll only achieve MV, at an enourmous effort on planning. When not having enough tanks and fighters, you have to fall back strategically, hitting hard and falling back at any critical point. There are places where you can hold the open with the mass of your core, and ambush with a Tiger II in a close pass (south of the starting front line f.e.). I remember PG1 had partisans in this scen, so I always placed my best infantry units backwards to wipe out partisans on turn 1, thus allowing free withdrawal to my core. I was usually able to hold Warsaw, and sometimes even Minsk. But cases where I achieved DV with a not ridiculous strong core, are few, very few. I focussed in not lossing core units, mostly sacrificing auxiliary untis when someone had to earn me one more turn for withdrawal. Because if you happen to fail at Batalon (likely to happen if rain stalls your drive), you then have to face the hardest scen: Berlin. In any case, the war against Soviet Union was the bloodiest and toughest of all wars germans fought. So it's fair that Bagration is that hard.