I used a vise and a couple of 2x4's to simulate a hard to reach pipe. As a plumber I encounter this a lot. They do make a cable saw for this but, they don't last. I'll show you a way you can cut it easier than a cable saw.
Step 1: Get Nylon String
The string on the left is nylon . It's very common string. The string on the right is not nylon it's cotton and will not work.