In this video guide I will show you how to knock 19 bikers off their bikes with the Phoenix's dodge. First off to make this achievement way easier your going to want to go to Wellspring in Torn Plains. Do Loosum's first mission so you unlock her projects tree. Once you go into her perks you will see one of the first ones will be for being able to track the convoys.
Once you have that perk you can go out and find the convoys by looking at your map. The ones on the lower part of the map are easier to take out if you want to go that far. If you feel you can take the damage go up to The Wilds. It seems like there are more convoys up there. Plus they seem to have a couple extra bikes as well.
Now once you have decided where you want to farm the bikes, all you need to do is get in the Phoenix and drive up to a bike and press the Y button and the LS towards the bike. You don't need to kill the biker or destroy the bike. Just simply knock him off. Every time you connect with the dodge he will wipe out and count towards the progress for the achievement. After you finish with one convoy's bikes head to another right away and repeat. You don't have to finish off the entire convoy. Eventually the bikes in the convoy you take out will come back after time. Once you get that 19th guy knocked off the achievement will pop right away.