IELTS Grammar: wide range or error-free?

A student, Kishanth, asked me about two of the points in the grammar criterion for band 8 writing (both task 1 and 2):

• uses a wide range of structures
• the majority of sentences are error-free

Here's the problem with these two points:
When you try to use a wide range of grammatical structures, you risk making more mistakes (errors). But to reduce the number of mistakes, you might have to simplify your sentences.

So what should you do?
My advice is this: Forget about the 'wide range of structures' point. If all of your focus is on using complex grammar, you'll probably do something worse than just make mistakes; you'll probably forget to answer the question properly. The people who worry most about grammar usually neglect task response, coherence and vocabulary.

For me, grammar is the least important criterion to worry about. I tell students to focus on the other 75% of the scoring system: task response, coherence and vocabulary. However, if you want my tip for improving your grammar score, I'd say that it's better to focus on reducing errors. I think examiners notice errors more than they notice grammatical range, and if you write a mix of short and long sentences, you'll probably fulfil the 'wide range of structures' requirement without realising it.