Hi, and thank you for buying our guide
If you look on that site at the
following page
(german version) you will find a framed text at the beginning of the page called "WARNHINWEIS". In this paragraph you will find information about the difficulty of this trek and you will see that most of the stages of this trek are described in the site (and in the book) as day hikes (Liste der relevanten Wanderungen für die einzelnen Etappen des Treks*). So if you use the listed day hikes description you will find all the information about difficulty level and much more.
Of course as you are going to do the all trek, and not only day hikes, you can consider reasonably that the over all difficulty is more then the sum of each day hike difficulty.
To make it straight the Long crossing is a difficult to very difficult trek.
You can also look on the forum for post about the Long crossing, there are quite a few people who have done this trek and they talk about it.
Have a nice trek in the Lofoten