Just grab the gem and then sneak into the Legion camp and power up the gem. Hope the fence on the right side of the city and sneak around the elevated walkways to your left. There aren't many Legion on the way and the ones that are there can be taken out with a sneak kill, provided you've purchased that ability. You're headed to the glowing portal in the southwest corner of the city. Stand in the middle of it and press
repeatedly to charge the amulet. Once its charged, climb the boxes against the wall near the portal and leap out of the city.
Now take it back to the ruins and power it up at the platform near the sea. Its near the Leystone and looks like a low, circular platform surrounded by rough stone pillars. Once the dragon has been summoned, you'll be instructed to pick a target for the dragon to destroy. Press
to access Aesir vision and you'll see potential targets highlighted by red circles. Target the boats and press
to have the dragon destroy the boats on the left. Once the boats are destroyed, the achievement should unlock.