This will probably come to you naturally during story mode (especially in the mission with the bulldozer).
If it didn't, the best way to get it is in Slaughter Mode with the Military Truck and that is for two reasons. First, the best way to keep a combo in this game is to run over zombies and not shoot them. It keeps the multiplier going as one of them is always going under your wheels. Second, you need tons of zombies in a close proximity - which is essentially what Slaughter Mode is. You should easily be able to get this on any map in the later rounds with the right vehicle (You'll need a lot of points to get gold medals on all arenas for the 'My Favorite Color' achievement.
The Military Truck is unlocked by obtaining a silver medal in the Driver's Cup (Blood Race Mode). You'll need a gold medal in all cups so earning this truck will be required for the completion anyway.