Difficulty: Easy
While you can probably do this at any big cliff, there is one in particular without any bumps, and a nice obstacle free path (about 12 seconds of sliding too).
Yngvar's Fang in Northern Skellige is the place. First you need to travel to the top (lots of running and jumping), where you'll find a chest, signpost, and a place of power. Here's your slide. Slide down from the mountain, steering carefully, and you'll have a trophy a few second later.
Here's a picture of the island in question: