This can be quite a time consuming achievement. But what I did was to tie it in with the Variety is the spice of life achievement for trying every car and track.
To setup a race go into Single Player and under Game Mode set it to the following;
Day Night - 15:00
Race Length - 5 Minutes
Time Multiplier - x1
Grid Position - 1st
Opponent Count - 19 (You can have less, I was using 19 to practice for career mode)
Opponent Skill - 80 (Lowest you can set the skill, highest is 100)
Opponent Agression - 80 (You can set the skill between 0-100 it's your choice)
Single Make - Disabled (I never changed this setting with any race)
Once you have done that you need to select your car, You have three different options 2018, 2019 or ICGT. Some cars are only in one category so make sure you select the correct category when choosing a car. I will both list all the cars needed (In year order) and give you screenshots of each category.
Cars needed to use are:
Jaguar 2012
Aston Martin 2013
Audi 2015
Mercedes 2015
Lamborghini GT3 2015
Lambotghini ST 2015
Nissan 2015
Bentley 2015
McLaren 2015
Lexus 2016
Honda 2017
Porsche 2017
BMW 2017
Reiter 2017
Bentley 2018
Ferrari 2018
Nissan 2018
Porsche 2018
Aston Martin 2019
Audi 2019
Honda 2019
McLaren 2019
Porsche 2019
Lamborghini 2019
2018 Cars 2019 Cars Intercontinental GT Challenge After you won the last race with the last car you need the achievement should pop. You can't see what cars you have driven so I would create a word document and tick off each car once you have won with it.
Also if a car appears in two lists for example the 2013 Aston Martin can be raced using the 2018 cars and 2019 cars, you only need to win with it once for it to count. Which should make things easier.