I have created a blueprint (custom race) which allows you to complete challenge cards extremely quickly. It uses the Ferrari 599XX EVOLUTION which is the fastest car in the game. From start to finish you should get this achievement in about 30 minutes.
Starting the Blueprint:
When in Horizon Freeroam, press
to go to the ‘Horizon Super7’ tab, click on ‘Challenge Browser’, press
and search for the creator ‘Mbhyat’. Select the ‘Achievement 77’ blueprint.
Completing the Race:
Once the race starts, hold
to go forward and finish the race. Each race should take about 3 seconds and then immediately press
to retry the race.
Repeat this process until the achievement pops.