Take the entire rootball out of the pot and put it in a huge tub of water (25-30 gallons). Work all of the soil off the rootball, with a hose or hose with a nozzle of some sort. Then just swish/shake the whole thing in the tub of water, and work the roots apart. Pull apart a little, relax, shake, and repeat. The point of the water is that it supports the roots and gives most of them an opportunity to float apart with the pull, relax, shake routine. It makes them sort of weightless, so they can float apart. Have done this many, many times. You are still going to loose some roots, and you have to work at it with some patience