I have realized (with some hints from users) that despite the wither arena being smaller now and such, that it was still possible to kill several of them in very short time. So I placed 3 mob spawners into the arena: 1 Zombie, 1 Skeleton and 1 Zombie-Pigman spawner. I also placed 4 open lava- streams from the ceiling.
In the first test, this setup is already proven to be much more challenging. There is a clear need now for people to go in organized groups. I would like to get feedback from some people once someone managed to kill the wither. Currently there is one inside, unbeaten. I want it to be hard, but not impossible.