You will get your chance to unlock this trophy/drive the Bulldozer in Mission 15.
In order to keep combos going, you have to run over zombies constantly, if you go a 3 second period of not killing a single zombie, your combo will drop.
This mission doesn't have any particular time limit, so as you begin in your base, just simply drive out, go full speed and drive into zombies, try your best to not slow down too much as a high speed will help you out a lot, and always go for zombies even if they're in a group of 4-5, a group of 4-5 zombies is better than no zombies and ruining your combo.
I killed a big amount of zombies (140 or so) then failed the combo, so I simply restarted the level so I could be put back at base, and go the same way as before as I knew which part had ruined my combo. so simply run over 150 zombies without having a 3 second period of no kills and this is yours.