Do as HighT3chR3dn3ck said for the Bob the Destroyer achievement
"Mission 15 is where to get this. Simply start it up and take the bulldoze to the left and follow the trails of zombies. Sometimes you'll have to hit an exploding zombie that's in between the mobs, but just try to not go in a circle or down the same street again."
I got over a 200 combo so when I broke the chain I got both Bob the destroyer and Combo master achievements at the same time.