All swampfolk (and tribals) do 35 extra damage that ignores damage threshold and resistance on top of a weapon's normal damage.

There's no in-game lore explanation for this, it's just because.

This means that, at any level - 1 through 30 - no matter what kind of armor you are wearing, from T-51b to Enclave Hellfire Armor and no matter what perks you have - that a single Swampfolk with a Shotgun at 3% condition can kill you in one hit, from twenty feet away, as long as the number of pellets that hit you times 35 exceeds your hitpoint total - which can happen very easily even at level 30.

Two out of the four regular Swampfolk enemies - who are encountered with the same approximate frequency - have more Health than Deathclaws, and have better unarmed damage with them. Again, there's no real reason than this other than to artificially steepen the difficulty curve.

This is one of the reasons I have a Love-Hate relationship with Bethesda. For every good idea they have, they have completely stupid half-assed ideas like that.