Zombie Derby 2

To unlock this achievement, you will need to fully upgrade a vehicle. My suggestion would be the Veteran as it is the first vehicle you claim after completing the tutorial. And it is fairly cheap to fully upgrade.

Here is the breakdown of upgrade prices:
Fuel: 140 > 160 > 170 > 250
Wheels: 80 > 140
Machine Gun: 120 > 130 > 180
Nitro: 120 > 130 > 180
Push Bar: 220

All in all, it will cost you 2,020 coins to fully upgrade the first vehicle.

How do you earn coins? By simply running through levels, killing zombies and hitting birds. Flips may also grant you coins as well, but the former 3 actions will net you the most coins. There isn't really a "most efficient method" to grinding out coins, as it is very easy no matter what.

But if you are looking to get through levels as quickly as possible and last as long as possible on your runs, I would recommend that you focus on upgrading the skills in this order: Wheels > Fuel > Nitro > Push Bar > Machine Gun.

This order is just recomemmended because these skills allow you to be quicker, go farther and push through more zombies without reducing your speed. Upgrading the machine gun once would be beneficial for shooting obstacles, but the order you upgrade is all up to you.

Once you fully upgrade your vehicle, don't forget to purchase a sticker for 250 coins to unlock this achievement:

Here is a 100% achievement walkthrough for those that may be interested. I start grinding for coins around [3:07]:
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