Every time you pass through a green dotted location (like Mandalia and Sweegy), you have a chance of getting a random battle. So keep moving back and forth over those locations and eventually you'll get into a battle.

And for Dorter (which can be pretty tough for new players), I'd suggest using two wizards and a priest. Since it's raining in this battle, the wizards' Bolt attacks will do more damage. I'd also suggest making Ramza a knight, and have Delita and Algus be archers.