It sure is hard!
Though thinking about it, once you have the Sommerswerd, the only hard fights in the original book were against Kimah and then pretty much all of Book 11 from the Chaos-Master onwards.
Book 17 was very hard too in pretty much its entirety.
Personally I would like to see the Masters of Darkness made a little tougher - this is meant to be the climax of the Magnakai series, but even playing totally honest and remembering not to use the Sommerswerd, it is still a pretty easy book. Perhaps Kraagenskul could be made stronger, or perhaps force the user to fight Gnaag?
Slighty off-topic, but how does the Sommerswerd blow Gnaag to bits? There is no sun from where it can draw its power. After all, you cannot blow Kraagenskul to bits with it, so why Gnaag?