As I said in-game, as OP as Germans are, particularly the Tiger and Blitzkrieg is. If you're going to use the ISU you need to use and position it like the heavy artillery piece that it is, not like a tank. Similar to using an elefant, if you'd rather not do that use the IS-2 instead. But the IS-2 is auto lose against Tiger Ace and is pretty weak vs Stugs. But against Panthers and Tigers it's great because of how easy it is to deny repairs to the Tiger after trading with it's huge anti infantry damage.
Forget AT Grenades because they suck, forget Guards because all the good Commanders have smoke that instantly negates it. Spam mines, and keep your ISU really far back. It has such long range that you can position it really safely far away and camped by mines.
ISU's are fantastic in 1v1's and are typically my favourite heavy call in because IS-2, and T-34/85's have hard counters that can just get completely shut down if the German is decent and builds the right units. Where as the ISU you instead lose if the German doesn't drive over your mines.
Here's an ISU replays, note how caution the position and defensive line of mines are.
http://www.coh2.org/replay/16103/pwnagemachine-vs-panzergrenadier1942
Unfortunately I don't have more ISU replays, I have had some great games with it on Semoisiky Winter but I don't save replays that are one sided because they're boring and there's no point posting them. I'd also recommend using the ISU to destroy all the garrisons, they will blow up in either 2 or 3 shots and with all of the garrisoned leveled it makes it harder for the German to micro there tanks around and find flanks without being shot whilst doing so.
Your ISU not firing could be fixed with better micro, if you attacked ground on or in front of the Tiger it would have fired rather than continually turning around trying to realign with the moving Tiger. The ISU is all about attack ground'ing.
As for the AT guns there of course is a delay involved when the gunner of the AT gun is killed and someone replaces him, then they also have to re aim the gun and fire. It's definitely more than it should be, but ultimately Ziss AT guns just like Paks are supposed to be long range AT and not short range. The real problem is that there is no short range infantry based AT for the Soviets like Germans have with Shreks. If Penals or Guards got Shreks, or Partisans with Shreks/Double AT rifle for cheap where available on the base faction that would be fine. But currently Soviet AT is just absolutely dreadful but hopefully should be addressed in the next patch according to Relic.