21.

Pre-Battle Setup: Equip Denim with the Fire Sword and give his Fafniel Sword to Harborym. This provides the 'Dragon Slayer' bonus to three party members.

Initial Positioning: Place Denim, Presance, Sara, Harborym, and Ghost in the back row. Place Sisteena, Forcas, Aloser, Byan, and Canopus in the front row. A suggested formation is D-P-Sara-H-G in the back and S-F-A-B-C in the front.

Opening Moves: Let Canopus wait. The enemy Wizard will attack Denim with a weak magic attack, and the Witch will likely attempt to Stun him. Move Sisteena three squares ahead of Forcas and use Vitalize on Denim if needed. Have Sara move behind Denim and target the Hawk Man on the bridge. Position Aloser in front of Sisteena and use magic to hit Leonard and the nearby Hawk Man. Byan should move forward to attack the Witch/Hawk Man/Leonard, potentially finishing the Hawk Man.

Mid-Battle Strategy: Harborym can move behind Byan and use 'Accuse' on the Witch. The Ghost should move forward and use magic to hit the other Hawk Man and Leonard, forcing Leonard to use Cure+ and retreat. Presance should heal Denim (position him three squares behind Sara). Use Denim to throw a stone at the Hawk Man below and then move him in front of Byan. Forcas should move forward and finish the Witch with his Crossbow (consider saving beforehand).

Finishing the Battle: Canopus can move below (out of Knight range) and kill the remaining Hawk Man with Tornado. The enemy Wizard may use an AoE magic attack, but Sisteena can recover the damage with Heal+. Focus on eliminating the biggest threats first, attacking from afar initially to take advantage of AoE magic without friendly fire. Ensure your entire party reaches level 21 by carefully choosing who deals the final blows.