I have some questions.
A friend of mine suggest me that use more advanced words at your essays; it has score! is he right or not? Do advanced words have advantages at scoring?
Using idioms has any positive effect?
Complex grammatical structure provides more score or not?
It’s a good thing if you can use academic words and various grammatical structures. However, make sure that you use them appropriately and efficiently. Otherwise, they will give you nothing but minus score. From my point of view, I think the way you use your vocabulary and structures to express your ideas is more important. Some people didn’t use many complex words or grammars but they can still manage to get 28 or above in the writing section. TOEFL aims at testing your ability to use English effectively in academic environments. As long as you can make examiners understand what you mean, it’s OK.
Besides, I think using idioms in your essay is really risky because they are less formal and could have different meanings in different contexts.