Eager to see if someone has the secret. I certainly don't have the secret.
But in my method I use a brass bar and hammer. The brass bar I have is 8'ish inches long and nominally 3/4 inch in diameter. [Note: Loosen all the nuts on the cone washers but don't remove them.] I place the bar between two studs and give it a few hits with the hammer. Then move to the next space and repeat many times.
Eventually, the intermittent shocks transmitted into the flange will loosen the cone washers. I check the cone washers with my pliers regularly because they can break loose where you can spin them with the pliers before the are visually loose. Occasionally, the cone washers may pop completely off when they let go which is why I leave the nuts on so a cone washer doesn't get lost.
Good luck