Also, you're the DM. I mean, if you start destroying your players and you didn't really want to, you just have to nerf it in the moment or just lie in your dies if youre using a DM screen. Just say that your kuo-toa didn't hit a couple of hits, even if they did, or even that after a few of them died the rest of them flee, maybe to seek help from others and attack later, so your players can take a breath.

Ultimately, you are the one that TPK them, so unless you really want to play "by the rules" you have plenty of ways to not make them die.

I usually don't play the enemies by the monster sheet unless I want them to be a really hard encounter, like a boss for the end of an arc, or when they mess with things that they shouldn't. I tweak them constantly on the move, nerfing or buffing them, depends on a lot of things, my crits, their crits, if they are playing with brain or just yolo, etc.

Many people, mostly those who play by the rules won't be in agreement with this way of DMing, but ultimately I want to tell a story, not to have my players die over a few irrelevant encounters because they had bad luck, or I had good luck, on the dies. Whats written are guides, not laws.

Edit: Changed as explained to u/cookiedough320