This Focus Stone is represented by the bottom-most point of the pentagram on the map Saturas gave you. Getting there is another story. There are two convenient ways that avoid crossing into Orc territory. The first is to climb the rock wall behind the Old Camp, where the ground dips to a pack of Biters, the path that Pacho warns you about leading into Orc territory. After climbing, you'll come to a ledge with Blood Hounds around the corner; above, further up he narrow path, is a look-out fort where you will find two or three Orcs standing guard. You should be able to take them by this point, and there aren't many. Behind the otherwise uneventful fort (for now ...) is a bridge that leads over a yawning chasm, to another mountain path where there are a number of Blood Hounds.
Going straight, beyond the Blood Hounds, and beyond an intersection, is another bridge, one with a Stone Golem on it. The Golem won't bother you until you step out onto the bridge. You can destroy the it for 1500 experience with a hammer or you can rush past him across the bridge or having climbed on the railing you can walk across safely.
If you start from the Sect Camp, however, there is another way to reach the Mountain Fortress: follow the path leading from the Sect Camp to the area where the beaches and the bridges are. Follow the trail over the beach area, past a number of Lizards, and (right) around the rocks until you come to a path in what appears to be barren land. There are Blood Hounds on this path and the path itself connects to the aforementioned intersection. Turn left to see the bridge and the Stone Golem.
At the base of the other side of the bridge, you will run into Lester. Exchanging greetings, he will mention wanting to explore the Mountain Fort now above you along the path to find an once-expensive contract that may regain its price once the barrier falls; if you agree to help him/get him to accompany you/etc., you will lead up the path, around, and to the courtyard outside the fort itself.
There are three levels to the Mountain Fort and all three of those levels are infested with Harpies. Against the mountain is a ledge-d area that is too high to climb. If you proceed into the Fort and take the left path, you will find a store room and, down the hall, a library. In the library, you can find a very obvious switch on the wall; activating it opens a secret room that is swarming with attacking skeletons. From the entrance, through the right passage, is a large chamber with a ladder leading up. Lester will automatically say that he will check upstairs and that you should continue to search the lower level, but there is nothing stopping you from climbing the ladder after him as soon as you get there. The second floor will have access to the third floor via another ladder across the room, but the ladder is positioned too low to get onto the roof at the top (you'll just fall back to the second floor). Follow Lester out onto a balcony where there is a specially-locked chest. If you talk to him, and you both respectively say that you didn't find anything, Lester will very obviously indicate that you will find something in the library downstairs. And you will find something in the library downstairs. The something is a key and that key opens he specially-locked chest next to Lester, containing the deed. If you give the deed to Lester, he'll give you a Scroll of Telekinesis, and say he is planning to stay here a bit longer.
By now you will have already noticed that you can see the Focus Stone platform and the Focus Stone on it from the balcony Lester is on, but there is no direct route there. If you haven't yet, there you go. Since you were given a Scroll of Telekinesis, that is the "recommended" means of completing this mission - manipulating the Focus until it is off the pedestal and falls to the ground below. HOWEVER, and while it may seem impossible, you CAN reach the ledge where the Focus is by jumping off the balcony at its furthest, landing on the ledge at its closest corner, and avoiding bumping into the mountain wall on your right. Acrobatics may help, as can potions for sprinting.
Finally, as to the Harpies that plague this area, a ranger with a bow or crossbow will most likely have an easier time taking them as opposed to a warrior or a mage (not to say that neither a warrior nor a mage would be able to hold their own ...). A Harpy has a lower tolerance for arrows and bolts (10) than for other weapons (50) or magic (50), unless of course, you use fire (0) in which case you should note that one Harpy has (200) health and you will be harassed by at least three Harpies at a time, even if you have Lester as a decoy. Prepare to burn through a few potions, yon mage and yon warrior.