To battle in Journey Online one has to create a battle formation that will bring him Victory!
Each formation can be created with 9 adventurers and inside it you have the front, middle and back lines. Depending on the monster type the formation you create can either bring you success or a swift defeat. Monsters such as the Ogre Boss only have a simple front line attack, so it is suggested to set your tanky adventurers in the front line. Your middle and back line are where your squishier adventurers can cast spells or attack with their weapons from a safe distance.
However there are plenty of scenarios where you will have to change your formation out in order to defeat a certain opponent with a unique strategy. Some monsters will prefer to hit your back line while some other bosses will only target your most middle adventurer. Be sure to always adapt to new possibilities if you fail to defeat a certain opponent.
You can always change your formation outside of combat so be sure to properly prepare for the encounter !