Actually, when I think about it, none of them really gave me much trouble. The Cosmic Luigi races aren't difficult when you realize that Cosmic Luigi actually chokes near the end of most of the races (people are too quick to press start and return to the observatory to initially see that).
As for daredevil, it just requires concentration and practice, I was able to beat Bouldergeist after 3 attempts under the daredevil comment.
Speedy wasn't very hard at all, most of the times given were fairly generous, and as for purple comets, as Mß said, most are just tedious.