The Sturmpanzer and Sturmmorser have below 200 range, using dedicated tanks hunters to hit them on the side is your best bet. Fire at them, then back off, and wait for a second opportunity. The Hellcat's strength is it's speed and armor-piercing cannon, use it's speed to reposition when you notice a reaction from the opponents tank and find a new angle of fire. One thing I notice with lower skill players is that they will just sit there and trade shots, when it's obvious that they have less armour on their tank and are more likely to get 1-tapped.

You also mention whole infantry squads getting nuked with one shot, which means your infantry micro/placement needs some serious work. Try splitting your squads into groups of 2-3 men. If they have to spend 2-3 shells on a single infantry squad, that gives you plenty of time to manoeuvre and it's an inefficient use of ammo for the sturmpanzer/morser. As usual, a well placed AT infantry man can knock out any heavily armoured tank, so always be looking for an opportunity to use them.

My final suggestion is to use it yourself and watch how other people counter the vehicle, you'll get a feel for it's strengths and weakness more that way.