WARNING: The following contains spoilers for Episode 11 of The Strongest Sage With The Weakest Crest, "The Strongest Sage Faces a Powerful Enemy," now streaming on Crunchyroll.
Throughout The Strongest Sage With The Weakest Crest, Matthias “Matty” Hildesheimer was used to being the most powerful and highly skilled magician for his age. As the reincarnation of the powerful ancient sage Gaius, Matty has millennia of experience when it comes to fighting demons. He has even crafted powerful weapons used to defeat them, including powerful ancient demons. For most of the series, Matty has defeated modern demons with very little effort. However, after encountering an ancient demon who knew him as Gaius in Episode 8, Matty realized that the limits of his four crests could be tested in unexpected ways.
To track down the ancient demon's location, Matty traveled to the heavily guarded town of Folkia to access the ruins containing a powerful device he invented in his former life. At the ruins, Matty locates the ancient demon, only to discover the demon is actually Zardias -- an extremely powerful entity that was sealed away thousands of years ago due to his unbeatable nature. With the demon about to break free of his seal and unleash death and destruction upon mankind, Matty realizes he needs to retrieve an ancient sword he himself built as Gaius to stand a chance at defeating him. Luckily for Matty, this ancient sword is located at the Second Academy.
On his way to the kingdom of Eis, Matty instructs Lurie and Alma to retrieve the sword while he and Iris keep Zardias at bay. Although Lurie and Alma manage to make it to the Second Academy unharmed, the demon arrives before they have the chance to retrieve it. Having looked forward to fighting Matty face-to-face, Zardias begins to test Matty's limits by unleashing everything he has on the 12-year-old boy. While Matty also gives it his all during his fight with Zardias, he quickly learns he's out of his depth.
One way Matty tests Zardias' strength is by having Iris in her dragon form unleash the powerful dragon's breath at full force. While this attack is deadly enough to kill any other demon, it fails to leave a scratch on Zardias' skin, disappointing him. Conversely, when Zardias unleashes his own attack on Iris, he nearly kills her with a single stroke. Upon focusing his attention on Matty, he unleashes a variety of attacks that Matty easily dodges -- all except one, which manages to wound him. Although the wound isn't fatal, Matty realizes for the first time that defeating Zardias could cost him his life.
In contrast with all of the previous demons Matty has fought, Zardias easily proves to have the same level of experience as Gaius, which immediately puts Matty at a disadvantage. Although Gaius purposely induced his reincarnation to obtain the four crests that would make him powerful enough to defeat ancient demons like Zardias, this line of thinking may have proven faulty after all. While the power of the four crests may be an advantage in close combat, it doesn't make up for a lack of experience, which is the other issue that's putting Matty at a disadvantage.
Despite his years of experience, Gaius also obtained all of his skills and knowledge when he was the bearer of only one crest. With four crests, his knowledge only gets him so far, as he is still learning how to use his four crests to their full potential. Since Gaius also skipped entire millennia to reincarnate as Matty, there are a lot of changes the world has undergone that he clearly missed out on, forcing him to learn his new world's history from scratch. Zardias, however, has not experienced that same massive time skip despite being imprisoned for so long. As such, he's had more time to progress his knowledge and plan his escape.
With so many odds stacked against Matty, whether or not he manages to defeat Zardias without losing his life in the process is a burning question that will hopefully be answered in the Season 1 finale.