For writing, we do short, focused, well-defined writing assignments. For example, I've been using the Evan-Moor Daily Six Traits Writing workbooks, that target one writing and one mechanics skill each week, in daily 10-15 minute assignments (three days of narrowly-focused skill work, one day of drafting using a graphic organizer, one day of using the graphic organizer to complete a brief written product, ranging from a sentence or two, at the lowest grade levels, up to a paragraph or two, at the middle school level). This is about as much as my DC can handle of actual writing work.
Separately, we do a few 20-minute sessions a week of All About Spelling, which includes explicit OG-based instruction in phonetic spelling strategies, word-level spelling on paper, and sentence-level dictation. The key for us, I think, is that all of the writing activities are short, focused, and at instructional level, which minimizes frustration. Progress in spelling and writing runs completely independently of other subjects, even ones that are closely related to writing, so they don't hold other skills back.
With AAS, our sessions are timed. We stop wherever we are when 20 minutes are up. It's also errorless. There are no assessments or grades. Just a growing stash of mastered word cards. If a word is spelled correctly, I show the correct spelling, or give DC a chance to find the error. Then we fix it and move on.
