The Arm is part of the boss Trent in Grandia. The Arm cannot perform any special moves and can only attack.
The Trent will be the first battle after Rapp joins the party, he will probably be weaker than Justin and Feena, especially when it comes to magic levels. Rapp should initially cast Burnflame, which he will only be able to cast twice before running out of SP.
Once again, Feena may take the role of healer in this battle but this shouldn't be needed. Have Justin and Feena utilise their strongest attacks and magic and the Trent should fall rather quickly. Beware, the Trent has an immunity to both Water and Earth based magic and in turn many resistances Forest as well. Fire, Wind and Lightning magic should only be used in this battle.