>> but Panther's main gun isn't destroyed.
>> Panther destroy T34.
>> Engineer repair Panther.
>> ???
In a usual combat, you would ram the panther from the position he is aiming at.
If his main gun faces you while raming, it will get destroyed.
You have to do the ram right, and it will always destroy the main gun.
(The same way germans can "attack ground" to face the gun away from the ram)