The best way to do this is using the throwing ability, you should get to grips with the grab and throw technique right away, in the first area through the city your first achievement should be 'Open air parking', grab a car with the
button then depress the
to initiate 'Aim Mode' and take out the Helo.
The same principal should be employed throughout the game with every 'grab-able' object you can find, but only use them against zombies and later Duskbats because harder enemies will take too long to die and the object will perish. Grab-able objects include: cars, trucks, buses, lampposts (street lights), parking meters, dustbins, tables, chairs, bookshelves, bombs (they count) and a few other miscellaneous things lying around throughout the campaign.
Do not kill any zombies or Duskbats unless it’s with one of these objects and this should unlock by the end of the Broken Stair level, very early on.
Also there are a few areas with a lot of zombies and objects, you can if u wish kill them all then die and rinse and repeat, kills will stack up even after death.