A silver usually isn't too hard unless you are trying for this achievement early on in the game. It's best to wait till you have your agility up a few levels to try these, then all but 2 will be a cake walk. With upgraded agility you will get forward thrust boosters (that you get use mid air by pressing in the left control stick) to improve your time as well as multiple jumps. At agility level 4-5 you should be able to do most of these easily. I'm at 5 now and only 2 have been somewhat hard for me to gold but all good so far now.