Once you've completed the story go to the Main Menu > Continue to reach the final area and a fast travel map. From here travel to Traps and Skulls and walk to the area where the rock is placed during Depression. Turn left from the rubble and travel up the hill until you reach a hillside clearing and you'll find a pine cone in front of a stone rock.
Take the pine cone which turns into a grenade. Turn back and make your way down the hill going left to rejoin the track which you need to follow to reach the stone bridge. Look for the third alcove on the right of the bridge and walk up to the railing to trigger the option to throw the grenade. Once the words turn yellow you can throw it and watch the explosion.
Continue on to the end of the bridge and provided you completed all story related achievements before tackling this there should be a portal in front of you. Go through it and use the fast travel again to reach the Ancient God area. Exit the mines through the bubbling water and head towards the dam control building. Go through the building and out the other side and you'll find steps to your right that you can climb up onto the beach whereupon you'll find a dead scuba diver and the sniper rifle for the achievement.