Most candidates who plateau at Band 6 in Writing Task 2 aren't failing because of weak grammar or a limited vocabulary. They're failing because their essay doesn't give the examiner a clear structure to follow.
Start with a four-paragraph skeleton
A Band 8 essay almost always follows a simple, predictable shape: an introduction that paraphrases the question and states your position, two body paragraphs that each develop one main idea, and a conclusion that restates your position without introducing new content.
Depth beats breadth
Band 8 essays pick one idea per paragraph and spend three or four sentences developing it with a specific example, a consequence, or a comparison.