Beat Stalingrad two times now, one was a regular victory, second one was very close to a brilliant victory, only one turn away from it infact.
It's a bit of a tricky thing. But once you get the gist of it, it does get easier.
First off you have to get the most of your armour and other mobile units unentagnled from the mess in the centre, use their attacks, clear out the clump caught inbetween your troops. Then begin pulling out the armour and towards the northern river. The bridge there is a good place to aim for. You want to make a breakthrough there with your armour and motor units, not the Italian ones, you have a different purpose for them. Once you've shattered the northern soviet lines and begin pushing your armour towards the northern flank of stalingrad, you send the Italian motor division towards the rail lines and cut them off there.
The Centre: While events in the north and the south (we'll come to that later) are taking place, you should hopefully see the soviets pulling away troops from to reinforce the northern and souther positions, thus giving you the opportunity to push in the centre, which is quite important since the centre will be the main supply line for the northern push as well. Failure to keep it up in the centre could result in the northern push falling apart. Plus it also helps tie up some soviets from bogging down your northern push.
The South: This one is the least impressive looking, mostly romanians with a few Infantry, a panzer and a schützen unit. But it serves an important purpose. It's going to act as a distraction to your opponent, drawing in forces as you keep threatening with an attack from the south, mind you, don't reinforce the souther supply point, always pull back to supply areas or use a supply drop if needed. Because it's going to be a ruse to draw in valuable armour or infantry away from the northern push which is the important one. But, you're also going to be sending that Motor division towards the southern supply point. Thus, strangling the soviets as you also cut the northern one.
Final effort. With the northern and southern points cut, and the centre giving you your own supplies. The Soviet army should be slowly withering becoming easy targets allowing you to get to stalingrad and handle the defenders there.
That's the basic gist of it
Hope that helps.