WARNING: The following contains spoilers for Episode 10 of The Strongest Sage With The Weakest Crest, "The Strongest Sage Works With a Team," now streaming on Crunchyroll.
Prior to the introduction of Giluas, Matthias "Matty" Hildesheimer was used to being the most powerful sage and the most highly skilled demon slayer. Despite his current age, Matty is actually the reincarnation of an ancient powerful sorcerer named Gaius. Since Gaius induced his own reincarnation to obtain the more powerful four crests, Matty's rebirth is far from conventional. As such, he retained all the memories of his past life and has been using his previous experience to his advantage.
When Giluas entered the picture in The Strongest Sage With The Weakest Crest Episode 9, he was the first person Matty fought who actually gave him a decent bout, but that's not all. Matty recognized Giluas' power levels -- especially his potential to get stronger. While Giluas ended up losing, Matty still supplied him with a textbook he wrote to help hone his powers better and become a more efficient fighter. Now in Episode 10, the studying begins to pay off.
In Episode 10 of The Stongest Sage, Matty, Lurie, Alma and Iris make it to the town of Folkia, only to discover it's been invaded by 32 demidemons. Though not as powerful as full demons, these still prove to be a formidable threat as they've managed to fully enslave the town's residents. It's not revealed what labor they're using the humans for but the demidemons still force their complicity by paralyzing their thoughts, which is a magical version of a lobotomy.
While assessing this new threat, Matty informs his team that demidemons have a hive mind and work in groups to compensate for their weaker powers. To slay them, he concocts a plan that uses the demidemons' strategic use of teamwork against them and begins by unleashing all of Iris' dragon energy at once to create a distraction. Once the demidemons have gathered around her, Matty, Alma and Lurie then ambush them, but only manage to kill the first 20 or so.
Before the group gets the chance to work their way up to the demidemons' leader, Giluas pays Folkia a surprise visit. Not only does he slay the remaining demons, but he decides to take on the leader himself without help from Matty. The latter acquiesces Giluas' request, giving him the opportunity to show off the new skills he's learned since fighting Matty in Episode 9.
During his fight with the demidemon leader, Giluas uses a new attack he learned from reading Matty's textbook: recoil sunder steel. While it succeeds in injuring the leader, it isn't enough to kill him. This is the first time in The Strongest Sage that a powerful spell Matty has taught someone isn't effective at slaying a demon in one go, showing how far advanced he (ultimately Gaius) is in his skillset. While Giluas manages to maintain his fight with the leader, he does so at the cost of his own body.
In addition to repeatedly using his powerful new attack to injure his opponent, the recoil sunder steel also weakens Giluas in turn. It takes several uses of this maneuver before Giluas defeats the demidemon leader for good, though it does not end up killing him. Shortly after his victory, Giluas himself collapses from having exhausted his power, though he also doesn't die.
Comparing Giluas' fight with the leader to Matty's fight with his subordinates, it's clear that Matty's superpower isn't the four crests he induced his reincarnation for. Instead, his past experience as Gaius is making the world of difference in all his fights if not even an experienced fighter like Giluas is capable of slaying demidemons with the same level of proficiency. It then begs the question if Gaius really wasted his previous incarnation for nothing -- or if the four crests were really worth the sacrifice he originally made to get it.
