See ya.
I mean, really. In the OP example I'd say it's on the player to recognize that while the rules say the ability should work the fiction is screaming that it working as written would be ludicrous, and thus not even try to use that ability in that situation.
The thing was 35ish feet tall, not some impossible height. A high level party fighting that size, while not routine, isn't exactly unforeseeable. And it's not so tall that "standard" tactics shouldn't work.
And if they don't, the DM needs to give the player some leeway - not gotcha them into wasting resources on something impossible.