Make your team consist of Seth and Kaim in front, with Jansen, Cooke, and Sarah on the back row. Then, get Anti-Petrify on to as many party members as possible. Now the first Arthrosaurus will be a pretty standard fight -- he'll either use physical attacks (which can cause petrification) or hit you with Heat Breath (400 damage to the back row, or over 2,000 to the front row...) It's a very dangerous move, but thankfully the battle shouldn't last long. Keep having Kaim and Seth attack; they're really only there to provide a guard for the back row. It's also a good idea to have both Kaim and Seth take a power drink, as that will roughly double their damage. Have Jansen and the other mages keep hitting the Arthrosaurus with Winda until it dies. The fight isn't that bad, but it's only a practice for part two, where you'll be fighting four more of them at once.