Is There Any Way To Become Unstoppable?

We wouldn't say unstoppable, but there is one way to become nearly unstoppable. To do this, promote five Dread Fighters to the class of Villager, and then promote those same five up the Archer Class Chain until they are all Bow Knights. From there, give all of them Killer Bows as soon as you can upgrade them to such, and then have all of them learn the weapon's Combat art of Hunter's Volley. These five Bow Knights of the Apocalypse should be able to take down almost any foe in the game, quickly and from a safe distance too.

You should try to support these knights with Taitana's Fortify Spell and Silque's Invoke Spell. Also have your two strongest offensive spellcasters on your unit, equipping them with Mage Rings so they too can attack from long distance. Finally, include Alm in your unit to destroy enemies with his Royal Sword's Double Lion Combat art, giving him every defensive booster throughout the game to make him a defensive tank as well.

If done properly, there is almost nothing within the game that can stop this battle plan.