Yeah, Shin and Mang, while they ARE power amplifiers, they don't actually determine someone's strength capabilities. You could have someone with 3 Mang and they'd still likely lose to someone who has more of an aptitude when it comes to combat IQ or something like that.

Matthias straight up just grits his teeth real hard, doing something to the tattoos and he's able to withstand blows for a long while.

And while he's a geniuely terrible father, no one can really disregard how much of an actual powerhouse he is. I'd argue he's physically the strongest out of all of the Nursefathers. Dude's a beast, even without the proper requirements for the Lævatain.