Jester David
Hero
Well, the level 20 barbarian us proficient in Str saving throws and likely has a 24 Str. So they only fail on a 1.RAW, I'd say never solo an Otyugh ever!
It's CR 5, but just a few consecutive bad rolls with your 20th lvl Barbarian, 999 HP and STR 20, and you're poop!
If they were full health and raging for the first minute, it would take 33 rounds of the otyugh spamming the barb against a wall to knock them to 0 hp.