Use the ordinary phrasal language Band 7 calls less common lexical items, while avoiding the decorative proverbs candidates mistake for idiom.
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Band 7 asks for less common lexical items. Candidates hear that as proverbs. It means phrasal verbs.
A: Every cloud has a silver lining, and where there's a will there's a way.
B: It took me a while to get to grips with the software, but it caught on quickly once people saw it working.
A has used two idioms. B has used two. B scores; A does not — and the reason is not that A's idioms are wrong.
A has produced decorative proverbs: fixed sayings that could be inserted into any essay on any topic, and which examiners recognise instantly as memorised. They carry no information about the thing being discussed.
B has used ordinary phrasal language that a native speaker would use in a job interview without thinking about it. It is idiomatic, it is less common than understand and become popular, and it does real work in the sentence.
One kind fits the sentence. The other sits on top of it.

One is load-bearing. The other is stuck on.
Twelve phrases sorted by whether a native speaker would actually use them in an exam answer.
Sort each item into the correct category
Three decisions about whether the phrase earns its place.
One
What you mean: the argument ultimately rests on one factor — cost.
Candidates: at the end of the day it's about money / it comes down to cost
The expert chooses:
Because:
Two
What you mean: the software was difficult and it took you time to understand it.
Candidates: I understood the software / I got to grips with the software
The expert chooses:
Because:
Three
The task: Task 2. You want to say the practice spread among younger users.
Candidates: it caught on among younger users / every cloud has a silver lining
The expert chooses:
Because:
Six gaps across a spoken answer, each wanting the phrasal version.
Examiner: Tell me about learning a new skill at work.
I had to a new scheduling system last year, which took a while because nothing about it was obvious.
At first almost nobody used it. Then a couple of the senior staff started scheduling everything through it, and once people saw that, it across the whole team within about a month.
After that it really — we went from two departments to eleven in a year.
I'd say it was , even with the disruption at the start.
I travel for work, so being able to check the rota from anywhere made a real difference.
Though honestly, I'm not sure I'm training other people — explaining things patiently has never been my strength.
In the end it all whether the thing saves you time or costs you time.
Answer a question using two phrases, and find out whether they fit or were inserted.
Speaking Part 2 style: Describe something new you learned recently and how it went.
Two, not four. The test is whether they belong in the sentences you would have written anyway — an inserted idiom is more visible than a missing one.
About four sentences. Exactly two phrases from this sprint.
An honest no is the useful answer here.