Procella Sylph
Procella Sylph is one of the special enemies to emerge from The Wedge in the latter half of the game. It, alongside four others collectively make up the "Five Lights from the Wedge". Out of the five monsters, Efreet Malum is the only one you'll beat during the story. The rest are hunted down as part of sub-quests. Each of these enemies also unlock trophies/achievements upon defeat.
Climb the Tower
You will hunt Procella Sylph during the sub-quest Farewell, Mage. The Tower of the Wind Spirit is at the ridge of Mount Dhiara. Climb to the top and you'll find Sylph there.
Defeat Procella Sylph
You will not be able to confront Grand Gnome directly. Instead it'll summon several waves of regular enemies for you to fight. Defeat them until Sylph Zloa appears and the real challenge can begin.
This enemy is weak to earth and has a weak spot on its chest. Ideally, you'll want to hit that but attacking from the front is dangerous. Strike from behind instead and only hit the core when the dragon is knocked down.
There is another opening you can exploit when the dragon is flying. Its belly is wide open so long as you don't strike close to the head. You can knock down the enemy with Rinwell's boost attack when it's casting artes. Or have Alphen do it anytime instead.
Once the dragon is defeated, Alphen will attack the core by himself. Destroy it to finish the battle. You will be rewarded with the Billowing Cyclone trophy for winning.