Killing a monster in the books (I only read the short stories thus far*) is usually a complex, multi-dimensional task full of preparation, using your environment, waiting for the right time of day etc. Late game encounters lose that feel. Geralt always prevails not because he's the best swordsman on the Continent, but because he's a complete pro with a great talent for analysis.
Well how could that feeling and m.o. be maintained for random encounters with no prep allowed?
Witcher contracts are set up this way, but what's a way to be consistent with the rest?
If anything, W.C. should be much easier than the M.Q.