Well, there is no "hard as possible" config per se, but there are certainly ways to increase the difficulty.
Note that the server might start dropping TPS or just give up if some settings are turned up too high.
Every ability has a few common options in the config:
Turn maxStrength up to increase how far up the Wither will scale in difficulty.
Turn minLevel down to change how soon the Wither will be able to use an ability
The spawning abilities all have options to control how many entities are spawned at a time, how long the delay is between each group spawned, and how many entities are allowed to be in the immediate area. (These are the same options as the NBT for a Vanilla Spawner by the way.)
The Lightning and Teleport abilities have options to control frequency and accuracy.
Most of the abilities have multiplier options, these are generally used to scale the relevant modifer as the Wither gets stronger. Greater numbers are usually stronger.
Now, the Death Knell ability is the most likely to crash your server if you turn it up too far, I've had several reports of that happening. If the Teleport is too high, the Wither is practically impossible to hit and it will rapidly become an exercise in frustration rather than fun. If the Lighting or spawners are too high you'll probably experience horrid client lag (and obnoxious noise).