Here's a project I did for our server for a Wither/PvP Arena. There are plenty of paths and structures to hide behind to fight while still being open enough for other players to find you if you stay still too long. The terrain is also destructible from the wither and rebuilds itself after each encounter thanks to a plugin built by xDeeKay.

The trees that I used for this can be found here: https://www.planetminecraft.com/project/fantasy-tree-repository/

Enjoy.