Bullet curve bullets count only if they KILL the enemy. If they stumble they do NOT count against this Achievement.
The enemies at the beginning you can kill with one curve bullet. Aim to the head.
Later in game, don't waste your curve bullets for the shotgun guys or the monks. Use shrapnel bullets for these.
The guys with the machine gun (lamp attached to it) to shoot around the corner you can kill with one curve bullet if the come out of there cover and you aim to their head.