Your using a bad rule most people don’t know about, which leads to a lot of bad outcomes, from a source a lot of tables won’t have.

I think it’s truly best for the game to just not use this rule. As in, it’s dumb. The designers clearly agreed, given they didn’t bring this rule forward, but did bring the diving into liquid rule off the same page. TCoE 170.

This rule breaks peoples intuition of how they expect falling objects to behave. It’s harder to dodge a larger falling thing, and that it would do more damage. And of course easier to dodge a smaller falling thing, that would do less damage. Object still follow that rule, per the improvized damage rules, but for some reason creatures don’t? If you take that farther, at some point you’d have to put a stop to it, or else you’d end up ruling it’s a DC15 to dogde a city sized creature creature landing on you. So, where is that line?

If I had known ahead of time that we were using that rule, I wouldn’t have done the above combo. If I wanted to something similar in this scenario, I’d use conjure animals to drop 8 giant owls on top of the BBEG, because size doesn’t matter with this rule. You rolled one 16, but you still have to roll 7 more times to dodge the other creatures falling on you. That’s a potential 80d6 (8*20d6 split between the creature and the BBEG).

In the brontosaurus scenario as above, I would argue the falling player has advantage as a sort of unseen attacker, which means your rolling that saving throw at disadvantage. And most high(er) CR creatures have low dex mods. Doing the math here, this will work 84% of the time. Even if there dex mod is +5, it will still work 70% of the time. Even without advantage, 60%. It won’t be enough damage on to kill everything, but on average you can still kill some creatures up to CR12.

RAW, with only 1 player making 1 attempt with, it’s going to work out most of the time. You can say “you fools, I rolled a 16” all you like, but that only potentially saves your BBEG from the worst version of this combo. That’s without players debuffing dex saves through something like hex, using multi conjure spells, or trying multiple times.

Imagine how you would feel if your players did this to every one of your boss encounters, or even just your BBEG. I don’t think you would feel very respected at that point. I know I wouldn’t.

I’ve never met a player at a table who knows about this rule, the community clearly doesn’t know about this rule, hence the above meme, and the insect plague meme recently, and the old fairy/polymorph memes. Odds are this is going to be a surprise for one party or another, usually both. And it won’t be a fun surprise for either. I’m sure you’ve gone over this niche rule with your tables, but I find it much more likely that players expect to work like described in all these memes and posts, and yeah, they are gonna be mad when it doesn’t work like they would expect it to.

That’s why I’m telling you to not use these rules at all, to just avoid the whole concept that has been problematic for the entire history of the edition. Otherwise the optimal build for every charecter is to get as many 3rd level spells slots as they can.