Got the achievement a while back but seeing as no one else has mentioned this method thought I'd put it out there. I would say it's easier than the other methods posted here and a lot quicker to setup.
Head to the Western Gate fast travel point in Oxenfurt, Velen.
Using the above image as a guide, go towards the building with the little pole on the roof (just visible between the tree and past travel post above). You should then be looking at these two roof's
Simple drop down and run up the roof on the left. Geralt will begin sliding, and jam against the roof on the right. Simply hold him there (may need slight adjustments to stay in position) for 10 seconds and jump off. Achievement yours. Hope it helps!