There are a variety of ways to reach the monastery region so I'll leave the navigation up to you. It is represented by the upper-right point of the pentagram on the map Saturas will give you. When you reach a place where a ravine is crossed by a log and there is a path leading down to your right, you'll encounter Gorn. Talk with him some and you'll learn that he wants to explore the monastery ruins himself; before that, he wants to check out the path to the right, just in case something is waiting in ambush. Tell him you shoudl explore together. Follow him (or lead, eh) down the path to the right and you'll find it passes through the aforementioned ravine. There are Snappers down here, so take them out and proceed through the cave at the other side under you come to the chest. Inside the chest you'll find, amongst other things, Transform into Meatbug scrolls, and you will be needing them soon.
Head back up to the top of the ravine and begin crossing over the log, coming onto the other side where a pack of Snappers will be descend upon you and Gorn. Go around the corner to find the locked gate that leads into the monastery, and a winch beyond that. Gorn will comment that the only creatures who have been able to get past the gate for years have been the meatbugs (there's two of them crawling around at your feet). Ignoring the stupidity of the visitors this monastery has seen in the past few years, use a scroll of Transform into Meatbug and pass through the small crack in the wall to the right of the gate. Use the winch to open the gate and talk to Gorn again, but don't try to access the winch as a Meatbug - that will cause a minor action lock-up that isn't easy to escape.
Gorn will probably, unintentionally, make you fight the park of Snappers in the courtyard (how did they survive in here for years?) and, when done, head towards a cave near the back left wall. More Snappers and a Shadowbeast at the end. Use the door in the wall at the end and open the chests to find the Focus Stone.
Now, if you hug the right wall all the way around the monastery until you reach the gate, you might accidentally miss this next trigger. I had Gorn follow me all the way over to the step region to the back of the monastery and didn't know what was going on until I went back down. In any case, if you head towards the door, you and Gorn will encounter a young troll. Unlike his grown-up form, you can kill a young troll just by hacking away at him. Once you're done killing, Gorn will tell you that he is going to hang around the monastery a bit. You can go on to your next adventure.
As an aside note, I mentioned the "step" part of the back wall of the monastery. While not really "steps," you can jump-climb this area until you reach a platform with a lone pillar and another platform area across a gap. Run into/hack at the pillar from a certain direction until it falls down and creates a bridge for you to cross over to the other platform. Your reward is a treasure chest of insignificant items and guilt for contributing to the collapse of a monastery.