When Asta and the other Magic Knights made their way into the Heart Kingdom, they found out they only had six months to train in order to be strong enough to defeat the Spade Kingdom Devil users. While the manga initially skipped over this training in favor of a time skip, the latest chapter of the series revealed a peek into this period and revealed just what kind of training they went through in order to get as close to the Stage 0 power level as possible. As it turns out, their training involves crafting runes.
Chapter 248 of the manga shows off just how much stronger Luck had gotten during the time skip, and revealed that a key part of their training in the Heart Kingdom was learning how to craft arrays of different runes (much like Gaja did in the fight against Noelle in the others before) to better harness the natural mana around them.
As was explained previously, the Clover Kingdom's strength comes from all the odd magic made from their unnatural relationship with mana. Because the country isn't so naturally receptive to the elements of the world, their magic comes from man made sources like iron, darkness, and more. So a big chunk of their training is learning how to wield that natural mana.
248 sees Gaja try and teach Luck and Magna about how to use the rune arrays, but Magna has trouble because he wasn't at least at Stage 3. Luck does this naturally, so he moves onto the next stage of training which involves setting up the runes so they "issue orders" to the natural magic around them. The more complex the runes, the stronger the magic.
Luck then uses this training to form his strongest attack yet, and it's teasing that the rest of the Knights trained in this same kind of fashion to quickly get their strength up. Now it's just a matter of seeing what kind of new strength each of them will have as they face off against their respective Spade Kingdom devil threats.
How are you liking Black Clover's current arc so far? Curious to see just how strong the Spade Kingdom is going to be as the arc continues? Who's power up do you want to see in action next? Let us know your thoughts in the comments or talk to me directly about all things anime and other cool things @Valdezology on Twitter!