Here's the feat:
"Born as the seventh descendant of the Bernille family, he was a titan at birth. His flesh and bones grew sturdier as he grew older, and since he turned seven, he hadn’t suffered a single injury. He possessed a disadvantage in not being able to use auras, but it didn’t matter. His firm body and muscles that could crush steel were enough for him to become a weapon of war when he was seven."
Kinda dubious on it, but since he is considered a weapon of war since he was young, and the fact that weaker character can already rip off fingers and toes no problem, and that fact that everyone was scared shitless of him the moment his name was mentioned and called him a mutant, I don't think it's that far-fetched.
First, we need to find the total surface area of the dude's body.
Kids at 7 years old range from 45 to 54 inches, but I'll go with 54 inches since he was considered a titan as a child. Dude's Height = 1.3716m
Dude's Ideal Weight, according to an ideal weight calculator = 32.7 kg (High-end, since he's a big boy)
Dude's Body Surface Area = 1.13 m^2
Now we can find the surface area of Dude's hand. The human hand is approximately 0.83% of the total surface area of the body.
Dude's Hand Surface Area = 1.13*0.0083 = 0.009379 m^2
The crushing strength of steel, according to google, is 400 MPa.
Force = 400*10^6*0.009379 = 3,751,600 Newtons, or about 382.426 Tons (Class K)
Of course, this will be a likely/possibly rating, at best.
Another Feat which is simple[]
Okay maybe not so simple but basically, a Hydra appears in the series, and its stated it walks on four legs, and that its height is 6 meters.
"He didn’t need to lift his head to know it was over six meters long. It didn’t seem like much if you only looked at its height, but it felt like a whole building was moving toward him. But, ironically, he wasn’t afraid. He was sure it was…"
The "Long" part is most likely a translation error, as the text itself says he didn't need to lift his head, which has nothing to do with knowing the creature's length, and the fact it directly mentions height.
The closest, well, creature I could find to resemble shit like Hydra in mythos is a Komodo Dragon, and those little shmucks, when standing on four legs, have a height of 0.6096 meters, according to google.
There is no doubt if the Hydra or Komodo dragon stand up on hind legs they'd be a fuck ton taller, but I'll go with these statistics for now.
6/0.6096 = 9.84251968... to the power of three = 953.496001... times heavier than a Komodo dragon.
An average Komodo dragon weighs about 85 kg, so that would mean the Hydra is 81,047.1601... kg.
81,047.1601 x 9.81 x 3 = 2,385,217.922502... joules (9-B)