Build a shield generator from RFtools, make about 2 stacks of the shield blocks and build a 5x5x5 box out of them
set the Shield generator next to it and set it to block all.
Set a DE mob grinder above the Shield generator and you can now kill them without them being able to get away.
You can then build a wither boss generator with Autonomous Activators or Block placers above a DE portal set to the center of the 5x5x5 shield.
Wither gets build, starts spawning, falls through the DE portal into the kill cube, De grinder kills them.
Just need an ender hopper or some other form of AOE item grabber.
Stars for days!!!