Hi,
Sorry to hear you're having trouble! Is Panda used to living with other dogs? Sometimes even the most confident dogs can become scared by having another dog in their territory, especially if Rambo's the dominant type.
I had a problem with my boyfriends collie cross girl, who despite being very confident and happy outside her house, threw a major sulky, aggresive fit when I took my two girls in after a walk, even though one of my girls is as confident as the day is long, the collie's glaring totally wiped her confidence and she was terrified!
Hopefully you'll find a way to get these two used to each other

, best of luck! Clare