BB gun? Flat out wouldn't work. Any kind of gun= bad idea.
Wire saw maybe. Shoot a messenger rope/wire with an arrow. A rope or wire on each end of the saw would give you all the length you need. But I doubt it would work. Unless the wire was tensioned like a piano string, it would mostly just deflect as you pulled the saw.
Scaffolding is what you need.
You can rent scaffold sections from a building supply company. If landscaping is in the way, set up scaffold on either side, using screw-type leveling legs (which you also rent) as necessary. Then "bridge" between the sections with aluminum scaffold walkboards ("picks" are available up to 24' or even more), lumber and plywood as necessary, and erect another section of scaffolding on top of the "bridge".
This would have to be done VERY carefully and thoughtfully by someone with experience in scaffolding. It's probably what your painter would do.
But get back to us and post pictures, as "enquiring minds want to know."
And Most Importantly, keep at least one firearm within reach, or holstered, so as to keep it FIREARMS-RELATED.
(In before the lock?)