Both Leonardo and Melissa have to drink Energizer in the first turn of the battle.

That's the only way to conquer that battle in this version (0.1.7). If you're on a difficulty level Normal and above, you might need to farming around a bit.

When there's a boss that seems like impossible to defeat, make sure to drink your Energizer! It's so expensive for a reason.

Energizer is so effective because it buffs your attack and strength for 5 turns, your defense for 3 turns, you agility for 4 turns, it increases your chance of counterattack/magic reflect by a whopping 60%, your chance of dodge/evade by 40%, you and your buddies will "cover" for each other, depends on who has the higher HP, you'll also cast the state "Ironwall" to greatly minimize the damage. Also, it will grant you 20% chance of HP regeneration and 10% chance of MP regeneration.