Instructions
Complete the main storyline. This will place the player in the game's post-game mode, where the Item World, Dark World and Land of Carnage are unlocked, making level grinding (replaying a map over and over to earn experience points) easier than it would be by replaying main quest maps.
Level your characters during the post-game. Replaying the first map in Cave of Ordeals is a quick way to grind levels, as enemy units are stationed close to the spawn point and are high level. Depending on your equipment, a party of level-1000 units will be enough to defeat Etna.
Start a "New Game +" game. "New Game +" mode allows you to start the main story from the beginning while keeping your levels, equipments and created character units.
Complete the first two episodes and the first three maps of episode three. With high-level units, the main storyline maps are completed quickly.
Begin map 3-4. After the cut-scene, the battle will start. Focus your attacks on Etna and use combo attacks to increase the damage dealt. After defeating Etna, another cut-scene will play, and the game will end with a special ending scene.