For this achievement you need to get the gold or platinum relic in the bonus level “Stormy Ascent” which is located at the top of Cortex’s Castle on the third island, although you can play it straight away. The level is easily the hardest in the game, and for the gold relic a time of 4:30 is required. A text guide will be near impossible to follow, so here is a great video of a platinum run by GarlandTheGreat.
Some general tips:
Be patient. The level is extremely hard and will require potentially hours of memorising the timing of every obstacle. If you cannot make a jump and hesitate, wait for a second, you have time to make a few mistakes and still pick up the relic.
For the part at 1:19 in the video where a set of platforms rotate in a circle, wait on the moving platform until the platform moving downwards is at a comfortable height to jump to (takes around 4 rotations of the platforms) and then jump to the one going upwards.
For the part where you have to go downwards down the stairs that retract (around 1:50 in the video), at the bottom run to the bottom stair to make the jump over the last spiky obstacle. If you have the mask still it might be worth just using that there.
Once you get to the top, there are a bunch of tricky obstacles. The set of platforms where two come out at the same time (3:02 in the video), you have to jump onto the first one, then jump just as they retract and you should barely make it to the second one.
The birds at the end will likely kill you multiple times (I was killed twice by them). Wait for the first one to be furthest away from you and that should provide the bounce to hit the other two and get to the end.
This achievement will be a challenge, so good luck and try to stay calm!