This achievement is basically a freebie, as long as you are ready for it when the last race in the last tournament (zombie driver's tournament) comes around. I had a fully-upgraded SuperCar, which should've been the cars you should've focused your spending on throughout Blood Race mode.
If you need money for the car or gun upgrades, you can keep playing an Elimination race (the first race in Rookie's tournament is a good one to use) over and over and stack your money. After the race, you can hit restart and just keep gathering money over and over on that level.
With an upgraded SuperCar and weapons, just play the final race in the game and win gold. Gold is 31 kills, so the 30 kills are a given. Not only that, but the level is easy and generous - I had 32 kills with 50 seconds left on the clock.