I've set up a wither rose farm in the end. Hopefully there'll be a warp to it soon.
To start, climb up the long tower of scaffolding. All the way to the top.
Watch the endermen run at the imprisoned endermite! Marvel as they fall to their doom (kinda)! Allow this to run for a minute or so. This is REALLY IMPORTANT.
After a minute, flip the switch. This allows the Imprisoned Wither "Do Not Kill" to start firing Blue Skulls at the Endermens.
After your chosen period of inactivity, flip the switch again.
Descend, by dropping or climbing, to the bunker, and travel across the water to the side of the mechanism farthest from the Wither. Drop into the little hole, through the doors, and your bounty should await.
That's it. Simple & Effective.