Instructions
Place earth type dinosaurs in your party. By this point in the game, the strongest earth type dinosaur in your collection is the Dacentrutus.
Wander around the Darkwood Forest defeating wild dinosaurs. Each one you beat grants you experience, and with enough experience, your dinosaurs gain levels. Your goal is to bring your dinosaurs up to at least level 20.
Proceed to the end of the Darkwood Forest. You'll encounter Rod here. Before you meet him, save the game.
Attack Rod's dinosaurs. A level 20 Dacentrutus will have no trouble tearing through the opposition, so long as Rod doesn't cast Lightening Field. If he casts Lightening Field, then it doubles the offensive power of his lightening type dinosaurs.
Cast Earth Field to negate Rod's Lightening Field. Not only will this nullify Rod's magic, but it will increase the power of your earth type dinosaurs as well.
Continue attacking until you've brought down all of his dinosaurs. Rod will admit defeat and give you the Stone Fragment.