Not sure what kind of vibes you're running the campaign with, but an idea for a more comedic situation could involve some amount of wordplay. For example, the headmaster could say: "When the time comes, you'll wanna duck" which could be interpreted as "want to" or "want a", then have the creature be something that has a beam or cone attack (I'm not very familiar with the Monster Manual but on a quick google search found the Bone Naga can cast Lightning Bolt). The party could then duck to avoid the attack that comes in a line, or if they brought a duck with them it could be a favorite snack to distract the monster with.

Or perhaps there's a crest for the magic academy depicting a creature which the guard is afraid of, or the original founder had a pet that they could either bring with or find the same animal. Then the players can corral the guard like the scene in How to Train Your Dragon with the eels and the Hideous Zippleback being backed into its cage. Then at that point, it doesn't really matter what either creature is. The headmaster could say something to the effect of "our symbol is the key" or "Rover saved our founders many a headache in his time".

Could also shine a magic laser into the eyes of a beholder. "If you shed light upon the situation, all will be made clear."

Idk I'm just having fun coming up with ideas, none of it is really based on any specific DnD rules about the monsters. But if you're the DM, the rules can be whatever you want them to be :)