I would say you need a track drive excavator that has a bucket with a grabbing thumb.
You would use the excavator to grab and place the "debris from the old roof and 4th wall that have fallen" so that the excavator can have a stable place to operate from.
The excavator's bucket/thumb can then be used to systematically dismantle the wall.
For a better understanding of what i am suggesting you can watch this YouTube video on building a stone wall to see an excavator at work. (I am available to come to France, s'il vous plaît. I am a handy man.)
Photo from ebay.co.uk