I wouldn't spent excessive amounts of time doing the hardest questions ...
My thoughts would be the exact opposite. The hardest questions test your understanding the most, so I would suggest spending a lot more time on them, rather than wasting time on the really simple questions. For the most part there really isn't that much difference between the harder questions and the average level questions. It's usually you either know how to do this question or you don't know how to do it.
Occasionally I did come across one of the harder questions that was just completely irrelevant (like the simultaneous equations question in itute). But these kinds of questions were quite rare.
Ive done a shit tonne of simultaneous equations to find time in projectile motion... its so stupid -.-" but apparently they are unlikely to be asked.
as far as the scheme is concerned it seems to suggest that you will be pushed through a lot of it by step by step questions, things like apparent weightlessness and projectile motion questions that can be easily spread over multiple questions.
its also like 46 pages long, maybe a lot of graphs?