What examiners actually reward

Lexical resource is one of the four marking criteria, and most candidates misunderstand what it measures.

It is not about difficult words. A band 7 answer uses ordinary words accurately and precisely. A band 5 answer uses a memorised impressive word in the wrong place, and the wrong place is obvious to an examiner instantly.

What is actually assessed. Range, meaning you do not repeat the same words throughout. Accuracy, meaning the word means what you think it means. Appropriacy, meaning the register suits the task. And collocation, meaning the words that go with it are the ones English actually uses.

Collocation is the one nobody teaches and the one that separates bands. "Heavy rain" and "strong wind" are correct; "strong rain" and "heavy wind" are not, and no rule explains why. English pairs words by convention, and an answer full of slightly wrong pairings reads as non native however wide the vocabulary is.

So learn words in phrases, never alone. Not "commit" but "commit a crime", "commit to a plan", "commit resources". A word list gives you a word you cannot use; a phrase gives you something you can put in a sentence correctly.

Avoid memorised impressive openings. Examiners hear "In this contemporary era of globalisation" many times a day, and it is marked as memorised language rather than as your vocabulary.

Precision beats grandeur. "The number fell sharply between 2010 and 2015" scores better than an elaborate sentence that is slightly wrong.