if you have saved the 7 mudokons then you know where the bird portal is at the end of the level. right next to the bird portal are two vendo machines. one of them is BOUNCE. drink the bounce and run to the platforms, then you are able to jump to the muds, throw them off onto the floor and carry them to saftey. you say that you've possessed and killed all of the sligs in the stage, but if you find that you haven't killed all of them, the second vendo machine happens to be an INVISIBILITY, so drink it and don't worry about it.
hope that helps you out.