Would Gilthunder qualify as a youngster? He was 21 in NNT so 37 now. And it didn't look like a lightning attack to me.
If it's Gilthunder then the title of the next chapter would just be referring to Percival. Rather than looking at it as a lightning attack think of it as Gil blitzing around while covered in lightning and using his physical strength to demolish the monsters. The house monster is being sliced in half by some kind of energy coming from below him, so it looks more like he was cut than hit by an ordinary projectile.
That said, I think Tristan is the more likely option. As I mentioned earlier, the mystery of the arrow still hasn't been resolved. Tristan in the Arthurian legends is known for using a bow that never misses its target, so he has always been a strong candidate for explaining the arrow. It's possible that it could have just been a gag similar to when Percival accidentally gave Donny a haircut, but I feel like that would have been addressed after the fight if there was nothing more to it.
Hmm, it feels intentional that there was focus put on the moon's appearance before the splash page showed our mysterious benefactor.
Good catch. I have to wonder what the significance of that was. The most straightforward interpretation would be that the mysterious benefactor is somehow connected to the moon, either him powering up because of the moon kind of like a reverse Escanor or the moon itself somehow responding to the benefactor's power. If the benefactor is Tristan then it could also be vaguely hinting at the duality of his existence. Where can you find a better balance of light and dark than during a full moon at midnight? If we instead take those panels more figuratively, then they could simply be visually depicting the tides of the battle turning as the good guys gain more ground.