Slitherine

In single scenario, there's way too much Russian armor, and too much of it is high quality. Something like 6 IS-2s and 0 T-34/85s.
Also complete air superiority makes this scenario a serious pain for the Germans, all of their infantry and artillery get smashed between the air force and abundant armor.

Per Wikipedia the Germans actually should have more armor than the Russians. (Before the counter-attack of course)

The Soviets used well-entrenched infantry in multiple lines, and also artillery used as tank traps.

(Not that I want more frustrating slogs of scenarios...)