Elizabeth is supposed to be the apostle of the goddess, her power is bound to be unique. To boot the demon clan seems to have had a rare combination of powerful members along with being numerous. Even now their force seems to be immensely powerful and they only released 10 of them. There are still god knows how many red and grey demons out there along with a demon king. The demon clan seems to be numerous and has more powerful individuals so far. Not a single human we have ever seen in the manga seems to even come close to any of the sins so far and hendricksen himself pointed out that meliodas had power which far outclassed zarathras. Basically the strongest human the manga has ever suggested is nothing in the grand scheme of things. Humans have had 3000 years to prosper and even now they seem to fall short of the demon clan. Camelot, a rising power was almost helpless to protect its borders against a 3000 year old dumb weapon... If the humans were alone in this then it would probably take every single holy knight in britania to defeat them and even that with difficulty considering how they have fared against hendricksen and albion.