The strategy I used was to go searching for car drops using the level 1 starter car. You are looking for level 6 or higher. If you find one camp next to it and wait for another player to pick it up. Immediately as they spawn honk and challenge them to a head to head and catch them off guard and get a head start because they are not ready. If you win you get the achievement.
Take their car and find a level 1 car and you can get Bully in the same game if you win.
The Bully achievement in Forza Horizon 4 worth 95 pointsEliminate a car at least 5 levels lower than you in a Head-to-Head race.