Don't think I'll ever get it either as the Seb challenge is too hard. Got bronze and got silver in one of them. 99% complete in the game though so when 100% that will be enough for me.
Got platinum a couple of weeks ago. Vettel challenge was the hardest to gold, followed by some of the AMG events. Its mainly a question of time rather than difficulty. The 1000 car trophy was a pain, as the UCD doesn't refresh with enough new cars.