Panther didn't back out. It limped past the guns and proceeded to blast them while i was trying to reposition.
What position were your ZiS guns in initially as the engagement started? If they were "flush" or parallel with each other and relatively close together (less than a screen length away), the Panther will usually win.
Ideally you'd be in a somewhat inverted 'V' formation, with the AT guns pointed at the apex of the 'V'. Additionally, if your position and map allow it, the ZiS guns should be slightly stepped or stacked, meaning that one is closer to the front lines than the other. There are a few reasons behind this: One, if the first ZiS gets engaged and overwhelmed, the second one can retreat in relative safety. Two, when the first ZiS engages, the second can be in a position to better get rear shots. The latter is especially necessary against a Panther. They may not be used much nor have the clout they did in CoH1, but they have excellent armor and are difficult to take out regardless.
Even an 'I' formation with 2 ZiS guns can be fairly effective, particularly if that's the only formation the terrain allows.
Also, I'd suggest mines. The Soviets have a nice CoH1-esque, all-in-one type of mine which I've found necessary for combat involving ZiS guns. Even as Ostheer I tend to throw down at least one Teller mine in front of a PaK that I may have built due to the necessity for AT and the lack of fuel for a tank. Obviously if it's a choice between mines (which don't have a guarantee of ever going off on the correct unit, or at all if nothing touches them) and an AT nade, the AT nade wins.
Besides that, I'd simply echo Kurt Jäger over there: Conscripts 'cause of the AT nade, Shocks 'cause of the smoke nade and flank protection. Throw that smoke nade right behind or on your ZiS, especially if that Panther is charging forward and you're realizing quickly that it's gonna bone your crew.
It has to be said that AT guns are really at their highest efficacy when they cover a narrow passage and have one or two open, secured retreat paths. If they
need to be used in an open area, it's a good idea to try and find some wall/tank carcass/shack to set them next to just to keep options open and making the possible circle strafes less effective. When in doubt, mine the flanks.
Quick summary of the above: Always support AT guns with a couple of squads, use AT nades and smoke nades often, mines mines mines, and position your AT guns so they can assist and augment each other.