Smart DM solution: 'The chaotic mass of falling elephants all attempting to occupy each others spaces bounces around randomly, like an enormous, suddenly violent game of plinko. Let me roll to see where they fall as you carpet bomb your own position.'

Evil DM solution: Fudge the dice so that they land on everybody BUT the BBEG.

Zero Fucks to Give DM Solution: most of the elephants land harmlessly around the arena, leaving a massive crater and a dust cloud. As it clears, the BBEG has caught one of them one handed and says 'Thank you, I forgot to bring a weapon.'

If you hate "well, actually" D&D guys just save yourself some time, downvote and move on. Anyway: #1: "Animal Shapes" can only turn targets into animals of up to Large size. (You could do horses though.) #2: It's also capped at 30 ft. range. (You could lower the height though) #3: Insect Plague creates an effect, not a creature, so the result is not technically a valid target. (I think a "Come on, dude" from the players would probably be justified though.) #4: If treated as a creature though, swarms are immune to polymorph effects generally. #5: Animal Shapes also requires targets be willing. How you obtain the consent of a swarm of insects is an exercise I can only leave to the reader. Biggest problem, I think, and possibly the funniest, is #6: Just what happens when that many creatures in that tight a proximity get ballooned up to Large size? PCs would quite possibly be hoisted by their own petards if you allowed that.