In order to obtain the One In A Million trophy, you need to take down the Hostile Chopper with the laser designator.

It's not as hard as it seems. It has a simple dodging pattern. Dodge, Dodge, Fire, Dodge, Fire, Repeat. The key is to vear towards the direction it's going to dodge, and bang. A forewarning: if you're too early, it'll dodge that too. It's all about timing.