This Focus Stone is represented by the upper-left point of the pentagram on the map Saturas gave you. The path up to this area is found beyond Drax and his buddy. There are Snappers up this path so be careful when fighting them. As a general note, Snappers seem to have a rough collision box so, even if you managed to kill the wolves in their small den behind you by climbing the nearby stumped tree after goading them then raining down death with a bow or crossbow, don't expect that to work here. The Snappers here can clip up the side of the stump just enough to hurt you eventually, or knock you off.
At the top of the path, head left and you will find Diego. Talk with him to learn that he's here exploring but there are a bunch of problems up head. The first problems you will find are a group of Black Goblins with spiked clubs. Let Diego run up to them first and attract their attention. That should make it easier for them to be dispatched, even despite their tendency to dart around quickly. There REAL problem Diego referred to is just beyond the goblins, namely an adult Troll. Diego says that he will distract the Troll while you work on how to defeat it. Both Diego and the Troll are immortal so they can wack each other as many times as they want without causing damage to each other. On the ground where the Black Goblins were crowded is a skeleton with two scrolls - one is a personal statement which will indicate how to defeat the Troll and the other is a Shrink Monster spell scroll that will help you defeat the Troll. Just the scroll while the monster in question is distracted and it will begin to shrink; when it is fully shrunken, the Troll can easily be dispatched.
On the right wall of the canyon, above on a ledge, you'll be able to see the place where the Focus Stone should be located, but the entrance below it that should lead up to the platform in shtu with a gate. There is a winch in the canyon, near which Diego will have moved after the Troll is dead. The winch is initially jammed, but Diego says that he should fix it and make it usable. Open the gate and climb the winding cave until you reach the summit where the Focus Stone is ... and where the Harpies are waiting. Harpies will spawn below, as well, where Diego was waiting and he will engage those. You might be able to lead your group of Harpies down to his group and get them mixed-up and fight amongst themselves or all focus their attention on Diego, who will, at this point, be sniping at them with his bow and arrow. If you plan to fight them, remember that Harpies have weaker arrow defense (10) than they do normal weapons defense (50) and magic defense (50), but their weakest is against fire (0).
Grab the Focus Stone and you're done.
It is possible to completely avoid the entire ordeal. All you need is a potion of sprint (helpful, but not necessary) and a scroll of telekinesis or two. Run past the Black Goblins and the Troll to the other side of the canyon where the winch is located (jammed). Jump onto the winch if you can. Ready the Telekinesis spell and look at the platform where the Focus Stone should be located - you should be able to highlight the stone and use the Telekinesis to draw it off the ledge, at the least. Go pick it up and run away. You can perform this trick at any point in the game.
For 2000 experience, I recommend using the Scroll of Shrink Monster on the Troll and killing it even if you aren't running the mission with Diego, and even if you feel greedy when you see the price tag attached to the scroll. If you kill it now, then another Troll will be spawned during the ULU-MULU mission later, and you can kill it for another 2000 experience. Considering the price, even if you have to buy the second Shrink Monster scroll for the other Troll, that's still almost as much in experience as the cost in ore. Rather decent trade-off, in other words.