Sonic Generations Challenge 1: Hardcore of Enemies
(Chemical Plant Zone Challenge 3: Aqua Shield Challenge) The third Challenge Act will unlock a new Skill once you beat it, and it's pretty useful for platforming. All you have to do is use the abundant Aqua Shield pickups to get through the level by busting through objects and reaching new heights by bouncing with the bubble you're in. An S Rank isn't too out of the way, either; you might even have 15 seconds left on the clock at the end of the challenge.
(Chemical Plant Zone Challenge 2: Which Switch is Which?) The chunk of Act 1 that this challenge takes place in has now been fitted with numerous switches, and objects that react to those switches. All you have to do here is activate them to continue through the level, so you shouldn't have any trouble here.
(Chemical Plant Zone Challenge 1: Tails: Toxic Skies) For this challenge, Classic Tails will help you out whenever you need to scale tall distances, or excessively long ones. Unlike the original games, Tails won't become exhausted after a short amount of time, so you can have him fly through the entire level if you wanted to. If you want to go for an S Rank, you might not want to do that for some portions of the first half of the challenge, because Tails may be slower in some places.