So their three strengths:

-“Invulnerability” -Shapeshifting/Mind Melding -Physical strength that surpasses any monster Sam and Dean have faced

Their weaknesses:

-Borax is basically more powerful holy water to them -Decapitation immobilizes them -They don’t have humanity or human creativity -Their viciousness causes them to get caught easily

Compare them to demons, who can do anything they can do, without needing to clean up a body afterwards.

They’re also literally just suped up humans, they have that creativity that Leviathans are jealous of, in fact, their creativity is encouraged in terms of torture, and it’s been shown that some demons train up psychopaths to prep them to torture in hell.

Point is, demons have the things Leviathans have and don’t have, that they want.

Demons may be weak in the fact that most lower level demons will walk straight into a devil’s trap, but it’s also really worth mentioning that a witch was able to completely paralyze a leviathan for a day, and take its super strength away for several, all of this without a single incantation (He didn’t even know what a leviathan was either).

Witches just siphon off some power from demons, so lower level leviathans have no shot against more powerful demons.

Lower Leviathans can’t do anything against higher demons, but lower demons can just get a bucket of cleaner and do the ice bucket challenge with whatever Leviathan they come across (Then rip their heads off with brute strength).

Same thing goes with monsters too, which are BY BLOOD, supposed to be significantly weaker than Leviathans.

All they need is some Borax and their brute strength to take out most Leviathans, and this isn’t even talking about Alphas which are the fastest and strongest things we’ve seen in the series, next to Eve (Who’s literally supposed to be the younger sibling of Leviathans)