I achieved that trophy on Battle for spratly map... and the easiest way for me was to plant the mines near the landing pad and after death camp near it to plant next 2 ones after first two hit (60-70% dmg only) the target. 2nd way to do it (its easy on Lost Islands) is to wait til transport chopper is at least ones hit by speedboat (that happens alot) and plant after it throws flares even on shallow waters... remember that you have to kill a player not to shoot down the chopper/jet , so if you lucky and the AA missle hits once enemy aircraft some of the crue will try to jump out and 2nd missle will might hit them...