Well I've got a first draft done but at the moment, its about 100 words over the limit. If you're having trouble writing it, its because you're trying to get it perfect on your first attempt. Just plan it out first of all, and think of what you want to include, and just write it, ignoring the fact that it doesn't sound at all what you want it to sound like. Perhaps you could plan out what you want to include in each paragraph, and leave the most important paragraphs (Opening, Conclusion) for the end. It then becomes much easier to read over it, editing one part, removing the other, moving that around, making it flow more etc.
Hope that helps.