Laser sight, foregrip and red dot. The sorpion is my favorite smg, and that combo works well to keep the recoil slight to moderate. It kills in 4 bullets and they come out fast, so aim center mass chest.
Here's how I did it. Aim high at the body and hope you get a headshot. It really is easier than some of the others when achieving 100 headshots as far as the kills to headshots ratio goes.
If you are using Toughness I'd recommend taking it off, you'll flinch more when shot, mots of the time upwards and land a headshot... Happens to me all of the time.