You would probably want something with with about 12-15 teeth per inch. Longer the better. Your aggressive saw would probably work . You could certainly give it a try. Cut a little less than you want to remove and you can make clean cuts with scissors or other after you hack it up. See link for a nice root saw. You would ultimately want something similar.

Here is a link that might be useful: root saw

Yeah that one has finer teeth. this is the one I bought has 10" blade. only about 7-8 tpi but razor sharp!

Thanks