Alright, so I'm thinking about this from a pure logistics standpoint, because a lot of writers forget to think about the practicalities. Slime demons are often shown as these amorphous, corrosive blobs that can absorb stuff and regenerate. But if you go by that logic, their biggest weakness has to be containment and separation. You can't really 'stab' one, but if you have a powerful enough force to split it into multiple pieces and keep those pieces apart, you've basically neutered it. Each fragment might try to reform, but if they're isolated in separate reinforced containers or magically sealed pits, the main consciousness gets diluted or trapped.
Think about it like a puddle. You can't destroy the water, but you can scatter it until it evaporates. For a slime demon, that 'evaporation' might be a slow loss of magical cohesion if its core essence is divided and prevented from re-integrating. I read a web serial once where the heroes beat a city-eating ooze by luring it into a canyon and then causing a massive rockslide, burying chunks of it under tons of stone. The fragments were still 'alive' but couldn't dig themselves out to merge back together. It's less about a heroic sword thrust and more about clever battlefield control.