Slitherine

I have asked a similar question before: I dont understand the predicted outcome of a clash between my 88mm and AI tanks...
This time a 296 exp +A3 hero 88mm against a Valentine III, 101 exp, suppressed and in a swamp.

Predicted outcome: -2 vs. -8

Again I find the 88mm outstanding AT-qualities not reflected here - especially at long range.

Have you checked the pre-combat log (ctrl-left click on the target)? That might be informative. First of all, AT guns are much better at defense than offense. Your gun is attacking into a swamp which is close terrain and has an initiative cap of 2 but does not give the tank the penalties it would receive if it were in a river hex. Your infantry might do better as it would attack the tank's close defense value. Of course, if you move your other 3 units up, you can increase your mass attack bonus. Lastly, although suppressed, as it still shows yellow, it may be only slightly suppressed and again, I'd guess your 2 losses would be due to attacking into close terrain with a not good at close combat unit.

Anyway, I'm sure someone else can give you a more knowledgeable and/or detailed explanation.

Comments above are correct. Being in close terrain reduces the defense from 5 to 0 on your gun so when the valentine fires back it will hurt.

Another lesson learned....