If you have enough materials to build a wither and want to fight it, please go to the arena to do so. To get there, you can go to spawn and then type /warp witherarena.
You will arrive in a safe room where you can gather with your friends. You can go in there with as many people as you like. From there you can go down a water hole and then walk over (follow the signs) to the wither spawn box. In that box, you can build a wither. In the rest of the arena building is disabled. This is the only place in on the whole server where you can build a wither.
Please do not barge in on people killing the wither. If there are people already in the arena, check with them if they allow you to join. Do not steal their kill. If someone spawns a wither however and fails to kill it, anyone else coming to the arena while nobody is in there can have it. You cannot claim it for the next 3 days just because you do not have time to kill it.
If you have proposals on how to improve the arena, please do not hesitate to post a message on the forum.