The Bargain

Go to Bowserstone Market

Stay back and pummel his glowing red points with arrows or bullets.

After meeting with Theresa, head to the Bowerstone Market by walking or warping. Now enter the pub and head upstairs to meet with Jeeves, who's in possession of the diary you seek. Pay him the 1,000 gold he requests...but where's the diary? Yeah, that scumbag doesn't even have it on him, instead providing you with a map of where he buried it! What a guy!

Head back to Bowser Lake and follow the golden path to where Lucien's Diary should be buried. The only problem is its covered by a very large tree-troll thing. Thankfully, he's stuck in place and has slow attacks, meaning killing him is a breeze. Just keep your distance and fire your projectile weapon at the glowing red spots on his body; most are on the front, but a few can be found on his backside as well. As for his attacks, simply roll away to avoid the rocks he throws and stand aside when he pounds the ground to avoid the resulting shockwave.

Once he's dead, dig in the spot he was covering to retrieve the diary, then return to the Chamber of Fate and give it to Lucera.