|Act III - Havens Above, Watcher outside the shop||Serena's Magical Mending + 100|
Head to Havens Above in Act III and look outside the shop to find a Watcher with a quest. The Watcher wants you to head to the Battleground and find a rare mineral known as a Harmonicrystal so it can upgrade Serena's Harp. You'll want to be at least Level 60 before heading there, the monsters in the Battleground itself provides plenty of Exp.
Head down to B9 by way of the Camp and make sure you're fully recovered before heading through the Ultimate Key door. The Crystalotl is a fearsome monster and can kill you easily if you underestimate it. Magic is useless due to the Snub magic it casts so resort to using Re-Vamped Multifeet from Jade, Sword Dance with the Hero and Erik's Victimiser under the effects of Divide. Be sure to use Hendrick to tank the boss with Immense Defense + Forbearance. Upon it's defeat you'll obtain the Harmonicrystal, deliver it to the Watcher to complete the quest and get a large stat boost for Serena.