I use an acetylene powered soldering iron. You need a very high power one since the copper soaks up heat. Solder flow is limited by the available heat. For bigger jobs a wore wheel on a drill cleans up the surfaces to be soldered, corrosive flux (plumbing type), then apply iron on one side of the sopper and bar solder on the other. the solder will flow only after the copper gets hot enough.

I would definitely suggest contacting a professional or a manufacturer of copper gutters. I have an old farm house and when I ran into trouble I contacted several online sources. I added a link below for your reference.

Here is a link that might be useful: Slate and Copper Sales Company

If you don't have a lot to do, how about "just for copper". It's a glue type material, but it actually "solders" the copper. It would be expensive for a professional to use it on every joint, but if you subtract the cost of the tools, and the time it takes to learn to do it right, this may be a practical alternative.

To all who might be thinking about that, you may want to consult a professional first.

Here is a link that might be useful: Copper Gutter Suppliers