probably the weirdest fight I’ve had in DnD was in DnD 4e, where one of the official level 35 monsters was a sentient planet - and the directions for running the combat were just “draw a line across the battlemap, one side is space, the other side is the planet. Characters can attack any space that is “planet” and the planet can attack from any of its spaces”
The weirdest thing Konsi has ever fought was a magical feywild jellyfish that could turn its tendrils invisible, and create illusory “lures” on the ends of them to ensnare unwary travellers.
The weirdest thing Razira has ever thought was an extradimensional echo of a kraken that didn’t exist.