1 Eleven: So what danger does an ochre jelly pose?
2 Mike: They dissolve your flesh with their acidic slime.
3 Eleven: Oh. I was going to suggest pouring the acid from the first vat onto it, but I guess it’s immune if its own attack is acidic.
4 Dustin: That would make sense, but... this is First Edition rules! That will totally work!

I looked it up and in the 5th Edition Dungeons & Dragons rules ochre jellies have Resistance to acid damage, which kind of makes sense. Whereas in the (first edition) Advanced D&D rules there's no mention whatsoever of them suffering any less from acid despite literally exuding acid as an attack form.