Autonomous activators, my dear watson!
Build a tower where Activators build the wither in the top. He is when he is getting ready falling down to the bottom. On the way down he can activate a tripwire. This tripwire starts Drawbridges with compressed (those that are witherproof) and close the top.
When he comes down its just to kill him. He can maybe be killed by catching his head in the compressed cobblestone. Or a netherstar generator gives damage enough to kill him.