Hey,
started using the ISU about 12 matches ago, and through free xp have placed on the track, engine and radio. 18000xp to go on the BL10 which is universally advised as the way to go. People say the BL10 MAKES the ISU, and I hope they are right. Unfortunately I am having a real hard time surviving. I have a lot of experience, and have good stats, but this tank just plays poorly. Big problem is the lack of camo, it is as easily spotted as the S-51. One shot, even with net, and Im spotted, not so bad in a normal vehicle, but the armour on this baby is poor for its tier. One shot off, then boom, either oneshotted, or artied to death. A couple of rules I follow with this sort of vehicle (ie unsurvivable)
1) Never try to hold a flank, you cant engage in a slugfest,
2) Fire once, and like artie, repostition
3) Oneshot mediums. A oneshotted vehicle cant spot you, and damaged one can. Sorry, but despite the damage dealing potential of this vehicle, you cannot safely engage the heavies, and only should when they are at <30%. As the other chap said, move up as a support tank, as the only way you wont die is to hit others engaging closer teammates.
4) If artie is in the game, you will almost certainly die. They treat you as a high-priority, softer option, and artie is no different to the others, they want easy kills.
Good luck with it, but it is no Jagdpanther, ferdi or T95...