Both the elephant and the ISU-152 only really have to be concerned about enemy tanks. Infantry gets blasted to bits by the ISU-152, and soviets have no AT infantry that can assault and damage an elephant. Flanking a ISU-152 with any german main battle tank will quickly result in a dead ISU-152. This is fine I suppose, as leaving your ISU-152 unsupported like that should result in punishment. However, flanking a lone elephant with soviet main battle tanks will just result in a huge amount of deflected hits, giving a german player a lot more time to respond and send back up (the elephant also has 1280hp like a tiger).
Elephant is very vulnerable to guards or shrek carrying squads. What you mean to say is that it takes a while to kill them. But who cares, they move slowly anyway they are not going to get away. The elephant needs heavy armor because it is a dedicated tank destroyer. And the rear hit thing is patently false. One t34 can defeat an unsupported elephant easily. The elephant gives the German player more time because they are not used for the same thing. Even a single conscript squad given enough time can bring down an elephant with at nades.