Before leaving the village, you'll get a tutorial about Reserve Rushes. This is the game's "super" or ultimate attack if you will. Each character has a different Reserve Rush attack, like a multi-hit, wide area attack from Fidel or a massive, large scale healing from Miki. They're that powerful to turn the tide of battle so use these skills in the right moment. You can trigger a Reserve Rush any time as long as your reserve gauge has a stock of at least one. You can manually set how much stock you want to use; the more stock you spend, the more powerful and potent your attack would be. Also, triggering a reserve rush as the 5th move with 200% Cancel Bonus will deal twice the amount of damage to its already devastating damage output.
Basics
R2 + X: Activate reserve rush
Hold R2, press L2: Toggle how many stock you wish to expend. This can't go over what you currently have.
Hold R2, press Left or Right: Toggle which character will activate the reserve rush. Take note of the word "Rush" under their names.
Building the Gauge
There are three ways to build the gauge, which will also yield various bonuses. Attacking enemies increase the pink portion of the gauge which stands for EXP bonus. Interrupting enemy attacks increase the blue portion of the gauge, which represents Fol (money) bonus. Finally, counterattacking enemies will increase the green part of the gauge, which increases the post-battle SP gain. Take note that if an enemy successfully does the same action against you, a corresponding portion of the gauge will be reduced.
Since keeping the gauge retains the post-battle bonuses, it's recommended to avoid using Reserve Rushes unless it's completely necessary. The gauge will retain its level regardless if you sleep at the inn or reload your game. The only way to reduce the gauge is by letting enemies interrupt and hit you or use Reserve Rushes. This mechanic gives players the chance the weigh the merits of having good post-battle bonuses from a boss battle or kill the boss easily with a well-timed Reserve Rush.
Who activates?
After unlocking the Reserve Rush feature, an additional option will be available in the setting. You can either set the Reserve Rush Scheme to Set Independently or Character You Control Activates. It's recommended to set it to the latter since it will allow you to conveniently switch to the character and ensure that he/she'll activate the Reserve Rush as you wanted.
You'll be back in Dakaav Footpath but things will be a lot easier now since you'll have Victor and his subordinates with you. Gunter is an efficient melee fighter while Hana is a very reliable signeturgist that's capable of healing and casting good offensive spells. Shortly after proceeding in the map, another subordinate of Victor, Daks, will join the party as well. You can use this chance to easily clear mobs and earn extra EXP, Fol, SP, and items or you can just head west and follow Daks to the Eitalon base's secret entrance.
Continue inside and take out the first few sets of enemies. You'll eventually reach an empty camp. There's a storage cavern to the NE of the camp with two chests. One of them is locked so ignore it for now. Examine the large boulder to the NW portion of the camp to reveal another hidden path.
Continue forth and take out more enemies along the way. You'll eventually the last remaining forces of Eitalon, including the boss himself. Once the battle starts, concentrate on taking out the enemies in this order: healers, spellcasters, archers, then the attackers, saving the boss for last. The boss has high HP and defense and his attacks can knock you down without too much effort. Once he's alone, you can work with your allies in building up combos and cancel bonuses. Once the boss is eliminated, return to Sthal to celebrate.
Return to your house to meet with the mysterious kid who has just regained consciousness. After the conversation with Miki, leave the house and talk to Victor. After talking to him, return to the house to take Miki and Relia with you. Head to the exit of the village and talk to Victor again to leave the village. You'll have to take the Dakaav Footpath to reach the capital. Before leaving the village, you can trigger a couple of Private Actions in the village.
Relia is a non-combatant but she's an equally valuable member. As a party member, you can give her non-combat roles and equipment, especially those that give bonuses to EXP, Fol, SP, and many more. At the early part of the game, these roles aren't unlocked yet but once you do, don't hesitate to dump them all to Relia so your combatants can enjoy equipping offensive and defensive roles/ equipment.