The Arm is part of the boss Gaia Trent in Grandia. The Arm cannot perform any special moves and can only attack.
Like the original Trent, this boss has 3 parts; the Flower, which uses Flower Laser, the Arm, which attacks in a broad sweep to hit multiple party members and the Gaia Trent itself, which can melee attack one party member and use a variety of stat-inducing moves.
The Gaia Trent has a resistance to all types of magic so attacking with the party members most powerful physical attacks makes this battle easier. If party members get hit by Flower Laser too many times, heal with Feena's Healer or Alhealer spell.