I've played vs the ISU152 since the patch 3 or 4 times and managed to destroy it every time without elefant too. I did noticed it had been buffed, but I didn't sit and cry in game, I dealt with it.
It's simple...
1) flank with decent tank or even better, a pair.
2) come up behind it (it turns to slow to get the tanks before it's too late)
3) gg
Most times this happened, the soviet rage quit because once he lost the ISU it was gg anyway.
;-)
Certainly it is possible to do this, but against good players this is nearly impossible. One AT gun, or a mine, or a lucky conscript nade, or button, will stop the flank. Also the only tanks reliably able to defeat ISU-152 before being destroyed are the Panther and Tiger. Both of these units are too risky to send in that deep behind enemy lines.
As a result the late game in COH2 is now ISU-152 vs elephant.
In short ISU-152 is beatable, but not easily enough. It negates infantry combat and veterancy, and is very, very difficult to kill with armor (excluding elephant).