The Slip Sliding Away achievement seems like it would be pretty easy to get. All you have to do is slide continuously for 200 meters. The difficult part of the achievement is finding the right area to perform the slide. Only a handful of places are capable of helping you perform such a feat.
The areas that you want to look out for are marked by signs that say no sliding. These areas are on high-up cliffs with long dirt trails leading away from them. One such area is featured on the map below.
This area is located in the Noventarmas section of Valle De Oro. Following the blue Guarilla Trails to the left of the character icon on the map will take you to the bottom of the hill you will be sliding down.
You will know when you have the right hill because when you are climbing up, you will see the no sliding signs in the image below.
While you are on top of the hill, travel a little higher up to find the Marksman’s Gloves. After that, take the plunge and slide down the mountain. Be careful of the large rock near the bottom. Hitting it won’t kill you, but it will prevent you from getting the achievement. Use the left stick to adjust the slide angle and make sure to hold the slide button for the entire length of the slide.