The ISU 152 is not op.
It is an end game tank that cost 1000 mp and it is similar to the tiger.
A few points :
-It is very powerful
-It has a good front armor
BUT
-Very very slow
-Infantery can kill it easily by flanking it and if it is alone, it always dies before being able to get out.
-Has a very very bad rate of fire : 15 seconds! If you decrease its power, you have to increase its rate of fire or it becomes useless. It is actually not really useful right now.
It is more a support tank, this means that if he only has a ISU 152 don't worry but if he has many tanks and an ISU and you have nothing this don't mean that the ISU 152 is op but that you did shit before. Remember that it is an end game tank that cost 1000mp.
In 1vs1, any tank kill it even a panzer IV so I repeat it : it is a support tank.
The flak comes in mid game. The only way to make the flak owning the ISU 152 is to have a spotter (then you see this tank coming at 1km/h) the canon will have the time to kill it. Moreover support your flak with paks extremely useful. And if you are PE, get marders.