Two krak grenades work. They tend to destroy the walls around the door and leave the door itself intact though =)
It is like irl!
When you bash strengthened doors, usually the walls around them get damaged first :> .
Oh, and everything can be now destroyed (with the right firepower).
Actually, external tower walls cannot be destroyed (corridor walls between towers apply here as well). Thus, rather use keys / multitools on those, to spare kraks.
And if you lack the firepower, persistence would also work.