I couldn't finish 'em. Looks like this's gonna get complicated.
The Watchbeast is a boss in Final Fantasy Dimensions and it is fought inside the Crystal Chamber, on the fifth floor of the Crystal Temple. It is the guardian beast of the Crystal of Lux, manifesting when it senses the crystal is in danger. Though Sol's party is unaware of it at the time, the Watchbeast attacked in response to Elgo's presence among them.
The Watchbeast will only perform two actions: a normal physical attack on one target and Blaze. Blaze is what the player needs to watch out for, as it takes out a severe amount of the party's HP. Diana should be ready to heal at every turn, if necessary. The Watchbeast has a weakness to Fire, so Elgo should use Fira, however Aigis should use the Magic Bomb Fusion Ability if he knows it, since it hits harder than Fire. Sol and Glaive can help by delivering physical attacks (it's a good idea to equip Glaive with the Metal Claw). However, this battle is scripted – when the beast's HP is down, it will use Soul Despair, leaving the party with single-digit HP. The battle will end and a cutscene will start.
Without Elgo's Fire spells, this battle could be tougher than the first. Dusk should focus on healing (every turn, if necessary) and Alba should use Fire spells, while the others attack. The Watchbeast will eventually use Soul Despair, leaving the entire party at single digits. Heal quickly and keep attacking, as using Soul Despair is a sign that it doesn't have much HP left.