You can work on this straight after the prologue. If you don’t naturally drive and gun your way through the map, the easiest way to get this Trophy will be in Nick’s plane -it’s armed with a machine gun and bomber sights.
Right after you complete the first mission from Rye and Son’s (Far South of the map) you’ll be able to start another by climbing into the plane. The kills you get from the machine gun and bombing runs count. Afterwards, climb back in and continue the kill count.