Saturas points you to the 
Troll Canyon for another Focus Stone, where Milton is holed up among the trolls. The catch is a huge troll you cannot fight head-on, so Milton brews a spell that shrinks it down to size, paid for in troll skins. After that it is a telekinesis puzzle and a broken bridge between you and the stone.
You will lean on magic this time, but Milton supplies the spell, so just bring the means to cast it.
- Healing potions.
- A decent weapon.
- Mana, with a spare potion or two to top it up.
- Room in your pack for scrolls.
Save before you tangle with the trolls, the big one most of all.
Head to the Troll Canyon, up toward the upper-left of the map, around the eleven o'clock mark. Climb the stairs on arrival and you run into trolls right away. Take down the smaller ones first and loot them.
Once the first trolls are down, follow Milton. He leads you into a small cavern on the right where it is safer, plays out a short scene, and explains that he needs materials from you. Before you leave the cave, sweep it for supplies, there are potions and mana items here that you will want for the fight ahead.
Step out carefully and keep your distance from the large troll at all times. Clearing the smaller trolls first makes this easier. You need three troll skins from around the area.
- One sits on the far side from Milton's cave, near some plants.
- One is by the large statue.
- One is on the way back toward Milton's cave, with a herb nearby worth grabbing.
Bring the skins back to Milton, taking care not to drag the big troll along with you. He is happy with them and starts preparing the magic. Wait for him to finish, then equip the spell he hands you and make sure you have the mana to cast it.
Leave the cave and line up the large troll. Lock onto it and cast the spell to shrink it down, which makes the fight a fraction of what it would have been.
Cast while the troll is walking away from you, so it does not come charging the instant the spell goes off.
With it shrunk, attack and finish it off, then loot the body for the troll skin and anything else it carries.
Head into the nearby room, where a door needs two handles turned to open. Work both, then continue into the large open room. Go straight ahead into the next area and open the chest there, taking everything, above all the Telekinesis scroll you need for what comes next.
For a spare, go up the stairs to the right, kill the flying enemies, and look straight ahead for a second Telekinesis scroll. A backup is handy if your mana runs thin.
At the top, cast Telekinesis on the bridge to drag it toward you. With enough mana you can pull it fully across, but even halfway is enough if you take a running jump. Once it is close, run and leap over. The fall is brutal, so commit to the jump.
On the far side, pick up the Focus Stone.
Return to Milton, jumping back across if you only pulled the bridge halfway, and watch the scene. Then travel to the New Camp, find Saturas, and give him the Focus Stone to close the quest.