The Trent is a boss from Grandia. It is encountered in the Virgin Forest. The Trent appears alongside Arm and Flower which together form the whole monster. The Trent body has the ability to cast Sleep Gas.
The Trent is the first battle after Rapp joins the party, so Rapp 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.
Feena may take the role of healer in this battle, but this shouldn't be needed. Have Justin and Feena utilize 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 to Forest as well. Only Fire, Wind and Lightning magic should be used in this battle.