How about the good old spell lesson, getting them to write their own gruesome ingredients to throw into the cauldron? This always works well, particularly with low ability. You could appeal to their senses by setting up starter activity stations around the room with different activities such as mysterious sound/visual clips, smells, props etc. Get them to jot down all the disgusting things they can think of on post it notes and share them with the class on a giant cauldron on the board. Depending on how low ability they are- give them a copy of the original spell with blanks to fill or a writing frame to help them structure their ideas. Hope this helps!