Hi Jumo,
The kimi challenge is the hardest one by far. If you're not catching kimi it's cause your laps are not perfect, so you have to try harder. I had to practice in the Time Challenge section of the game for a few sessions before I came close to catching kimi. The really hard section is eau rouge, which you need to get through in 6th gear, and the following three corners after that. That's where you make or break it. Anyway, good luck, study the fast laps on youtube, and practice in the time trial section. Good Luck.