In the Phoenix you can preform a Dodge by pressing
Roll up next to a bike rider and press
while pushing the left stick towards the bike and you will knock the rider off.
Do this 18 more times for the achievement.
The rider does not have to die for this achievement.
TIPS:
You can also Boost with
to get close to the bikers.
Bikers can be found in convoys or randomly on the roads in the world.