I took French as the second language to obtain more points. So I only took the test once.
I studied french more than 15 years ago. Back then I was an advanced student and spoke quite well, having spend some time in France.
I took 10 weeks of private lessons, 3x a week, before the exam. It was hard to revive the language and the speaking part was for me the hardest. Speaking sessions comprises of 2 parts. The first you read an add and call to inquire more about it. The second you also read an add, and you have to convince a friend to take part on something mentioned on the add. The second part last longer, and it was very hard to find reasons to motivate the person, which I though to be very antagonizing (I had to convince her to volunteer on an animal shelter and she flat out said I hate animals and I am allergic). Anyways, that was the most difficult part and as expected my poorest grade.
Reading and listening I thought to be VERY easy.
Writing is not difficult either, but it takes practice, which I didn't really have.
With these 2 materials I suggested you will understand the way TEF works, which is very simple. Markings are now based on right answers only, there is no more punishment for wrong answers, so you can go ahead and guess if you don't know. If you take the regular TEF, there is a Lexique part which will be graded but it does not count for Express Entry .