If you are having troubles getting through the waves a few tips; go for the big truck full of barrels ONLY when many of the little ones are nearby to potentially take out the whole herd in I one blast. Prioritize the one with a red arrow over it when this occurs as this is the one that's is going to attack YOU. When you have the green rockets, they fly straight so either be sure to be lined up behind a bunch of them or aim into a corner a little bit ahead of them. When you have the purple square mines that you chuck, they go to the closest one so try to use those also when theyre all clustered together to maybe take them all out. Using these tips will make getting through the waves quicker and you can get more of them in each race.
This will also help with the other achievement to take out 1000 takedown opponents get completed quicker.