Actually, In gen 6, Bronzong gets the dark and ghost weakness.
Without any mold breaker or gastro acid, etc., ground type moves wouldnt effect an levitate Bronzong, but if a ground type move actually hit Bronzong, it would be super effective.
Bronzong only has 3 Weaknesses: Fire, Ghost and Dark.
Hope I helped!
EDIT: Yes, Fire is the only weakness for a levitate Bronzong in 5th Gen