Woot is alternative solution to the traditional mob farm in Minecraft. Instead of spawning mobs and creating some method to kill them and then collecting their loot, you can now create a mob factory. The big advantage is that is doesn’t spawn any mobs ? WHAT! The basic idea is that you create the factory structure, then supply it with enough RF/tick to generate the mobs. You also provide an inventory for the factory to put all your lovely loot into. You can apply upgrades to the factory to increase spawn rate, add looting etc.
In Woot mod, you build your factory then program it with the mob of your choice. Then you give it some power and an inventory to put all the loot into that the mob would have generated. There are multiple tiers of factory to handle higher tier mobs. So yes, you need the big factory to spawn the Wither! To make life a bit easier, there is a layout block to show you where you need to put all the blocks in each tiers multiblock.
You can also upgrade your factory to give extra benefits, all of which cost extra power:
Go to Woot Mod Wiki page.
For Minecraft 1.9.4
For Minecraft 1.10.2