Struggling spellers tend to think of short words as easy and long words as hard when it's often the little words that require more memory. The best way to convince a student of this is to use the TST 'building with puzzle pieces' lesson rather than going right into spelling. Building is dealt with in detail in the book, but, essentially, you start with pronunciation (everyone, not just the teacher), count syllables, then reveal the puzzle pieces - not before or you'll just have a puzzle building competition and not a spelling lesson. Alongside the visual clues, use the regular spelling lesson from the orange photo.
Here are the three words which came out of a TST workshop with the mum of a 13-year-old. Click on the photos to enlarge.