Your vocabulary is the key to doing well in the exam and the more the better.
Don't waste too much time learning idioms because they are difficult to use in
the right context and it is easy to make mistakes, but there are a few easy o
nes that you can learn and use, especially in the introduction to your task 2
essay.
To play the devil's advocate is one of them and I have explained what it means
and how to use it in this lesson:
https://ieltsintaiwan.wordpress.com/2016/08/16/ielts-vocabulary-idioms-for-int
roductions-to-task-2-essays-play-the-devils-advocate/
The page contains links to lessons about a few other very useful idioms
Please join my Facebook group to discuss it:
https://m.facebook.com/groups/767699863305697/