1. Know what the examiners actually score

Task Achievement at Band 7 requires: all requirements of the task fully covered, a clear overview of main trends/differences/stages, and data clearly selected to illustrate. Note the word "clear" twice. Miss the overview — the single sentence (or two) summarising the big picture — and you are capped below 7 no matter how good your English is. Most 6.5 scripts have strong grammar and zero overview.

2. The 4-paragraph spine (use it every time)