BossTweaks is a plugin that makes the Ender Dragon and Wither fight more difficult in several ways. It has a primary focus on the dragon battle, but still makes small changes to the Wither fight.
Tweaks to the dragon battle:
Tweaks to the Wither battle:
Both affected boss battles can be toggled within the config file. TNT is also disableable as it tends to leave a fairly big scar on the end island which is unlikely to be desirable on public servers.
Updates can take quite some time to be approved, the most recent files can be found on Spigot and Jenkins.