Just wait until the two helis are as close as possible to each other, then fire the Valkyrie (R1), wait until the rocket is between them and hit R1 to detonate the rocket.
If you fail and destroyed no heli: Try again!
If you fail and destroyed only one heli: Grenade on the floor, stand in it, die and try again from checkpoint!
If you succeed and destroyed both: Well done! Trophy will pop!