From what I've read, the single player campaign is really not worth playing through. Red Orchestra 2 had rather bad optimisation upon release along with bugs.
The optimisation is... better, though the new Rising Storm maps don't play well for me while I can play all of the original maps. It is really a multiplayer game.
From my playing, you need to be patient. Go prone and crawl, run and dive into craters, take cover whenever possible and don't stick your head up if you know there's enemies in front of you. Squad leaders help advance through smoke grenades (the enemy can't shoot what they can't see). I really did think it captures what combat would be like. Everything's going well than suddenly you're shot because you moved ahead just a bit too far or went around that corner without knowing there was an enemy there.
As a rifleman, you're guns good for sniping but really you need to push forwards. Take note of where the next patch of cover is (Rocks or a hole, walls can be shot through if they're thin) and move forward when there's cover fire. Lob grenades when you're near entrenched enemies and charge in soon after. Stay near you're squad leader as he gets new spawns on top of him, you don't want to venture out alone so stay with a group.
Edit: One things I've found useful is if you are sniping from a window, stand back from the sill. About a meter back from the window, don't take cover right under the window. The further back you stand, the less you're exposed to additional enemies other than the one you're aiming at.