I've tried the solutions above, but being on the last position often gives you all-star abilities when you pick up an item. And that's not easy to kill zombies the right way with. I don't know if just running over them counts for the 100 kills, as without the all-star power-up it doesn't, or if the smash the pilots do when we press A also counts. I don't know.
So what I did was to set up a game of two players, and put a second controller as the opponent. As the time is infinite, just drive Zobio and Zobiko to the set of items where zombies are a bit far forward, stay on one item and sort the power-ups you get. If it can't be thrown forward (rainbows, shields...), just use it and the bonus capsule will appear as soon as you launched the previous power-up. And if you get a missile, a bomb or a star, just go close to the enemy in front of you and launch the weapon. Horns work too.
I hit 100 zombies this way. Stay on the capsules, sort, and kill the enemies with the appropriate weapons. Took me a few minutes. And no opponent disturbed me or no time limit was able. Oh, and don't forget to put the obscales on, otherwise no zombie will be on the track.
Hope this will help some of you.
Don't forget to finish the race after you got this achievement, for the game to save your progression. I first quit just after getting it, but now I have to redo it because the achievement did count on my gamerscore, but not for the game. Check your achievements through the game menu, it will say that it is still unlocked (that's a secret achievement, so it will be notified as "hidden"). And it needs to be unlock in the game for the last achievement of the game to pop properly: 'unlock every other achievements'. And it's the same for all achievements. FINISH THE RACE AFTER GETTING AN ACHIEVEMENT, DON'T QUIT IT THE SECOND AFTER!!