Not sure what I’m smoking, but it is gooood! My StuG win loss record: 58% wins / 38% losses. No gold ammunition used.
Seriously you don’t have to be the hero that brings down the big baddie in 1-2 shots to make a very valuable contribution to your team. Sometimes delaying the enemy heavies is enough, if your own heavies are busy tearing up the enemy on another part of the map. Sometime you lose regardless of what you do, but generally a StuG with the top gun should be able to do good damage against almost all opponents, and make a very solid contribution to winning the match.
The StuG also has very good maneuverability which means that after taking a couple shots, it can displace to another firing location of its choosing. So for example when facing overwhelming enemy force take some shots, to force the enemy tanks to stop, then back away and move to another firing position. Repeat the process until either help comes, or hopefully your team (which presumably is mostly somewhere else, or you shouldn’t be alone) beats up whatever they are facing and caps.
StuGs 75mm KwK 42 L/70 does 138mm of penetration, Tiger I hull is 100mm, T29 hull is 128mm, IS hull is 120mm. So it can definitely penetrate Tier 7 heavies. On most but not all tanks your best bet is to aim for the hull. Tier 8s can be a bit harder, and you’ll bounce some or need to get a side shot. The M6A2E1 you mentioned is Tier 8 and has a very thick front hull and turret, but it’s side hull and turret are pathetic (44mm hull, 89mm turret). AIUI StuGs should not be getting matched against Tier 9s.
With Tier 8s I sometimes switch to HE. I figure if I can bring down the health of a tank 3 tiers above me by about 30% (say 5-6 shots of HE) that is a very solid contribution to my team. Hopefully your teammates are also chewing it up. Or if you’re alone, that 30% you contributed will make it that much easier for a teammate out the Tier 8 later in the match.
Generally IMO your best bet as a TD is to stay back and snipe at things. You may not be able to penetrate with every shot, but if you can hit the enemy 5 times and penetrate 3-4 times before it can even see you, you’re in pretty good shape. Also a lot of times if you’re sniping from far away, and the enemy isn’t aware of your presence they’ll turn their sides towards you and that makes it even easier to penetrate.
There are some specific weak points that you can target, although I find it easier to do so with a fast medium than with a TD, since I mostly snipe from far away with a TD. If you get a chance shoot them in the rear in the engine compartment (admittedly not very likely as a sniping TD), this often starts a fire. Also some tanks have crew access hatches on the rear of the turret (like the large boxy KV-2 turret). You can also blast viewports. A lot of people recommend hitting the turret ring, although I’ll have to say, I haven’t had that much luck with it. Also if you’ve got company with you, shooting a tank’s tracks so they stay visible and vulnerable and your teammates can shoot it is also valuable, this also sometimes makes the tank turn sideways, exposing their vulnerable sides to your team’s fire.
I’m hardly the best player here. So hopefully some of the real experts can chip in with their advice. The official Basic Tactics page is quite good.
IMO TDs should rarely assault. They should mostly stay on the defensive. There are some specific situations in which they can advance. They work well in narrow corridors like city streets where opponents are forced to come straight in at them. Keep in mind that if you can be flanked as a TD you will be in big trouble. The StuG is very fast and can pull things off like sneaking behind enemy lines and base capturing or taking out artillery, but you should be very cautious doing this as if you are caught by an enemy medium tank at close range, you will be toast. This is even more important with other TDs. The StuG is able to turn quickly to keep facing an opponent, unless it gets tracked. Most TDs can’t. But even with a StuG if your enemy tracks you while you are in the open they will then destroy you while you can’t shoot back.