Stug was initially for infantry support. Later uparmored versions with long guns did ok against tanks, but like was mentioned from covered positions.
You want to see real counterpart to Su85 you look at Jagdpanzer4.
Ugh at the cost getting dragged into a pointless debate.. no. Go look at the armor thickness and the penetration power of the two tanks. They were almost equal. The jagdpzr4 (well most of them) had the same cannon as the panther, so its gun had much more penetrating power than the 85mm on the su85.
If you don't want to believe me, just go look at the real world specifications of these two tanks... but it doesn't matter anyway cause this is a video game. Relic has thrown out realism for balance/game mechanics so many times it really is just a waste of time to argue about this.