Try to hit them from the rear or sides at steep angles, 60+ deg.
I pumped 10 (TEN) 75mm AP from a PzIV into an ostie at 100yards yesterday, then he blew.
One 75mm in the side will kill them everytime.
Mg151/20 or MK108 seem to have no effect on Osties. Prolonged 7,92mm fire will tear them up, as will Browning M2 .50s.
The best method for killing these guys from the air seem to be 500kg bombs (hey, thirty, nice fitin' sunday

)