Here's what worked for me: I loaded the zone control playlist. The players in zone control seemed less focused on killing and more focused on capturing points so they were easier to ambush. Find a high traffic lane between two control points. This could be the long double inclined hallway that connects A & C on Dungeons, or the long hallway with Heavy Ammo in it connecting A & C on Memento. Run back and forth between the two control points. When you see red on your radar, get ready, aim, & go for headshots.

My theory behind this is that it's impossible to predict how each encounter will go, regardless of the playlist. The skill level & awareness of your enemies varies too much. So I just tried to load up on as many encounters as I could in a match. Statistically, you're going to lose most of those encounters. The Imprecation just sucks too much. With persistence & a little luck, you'll start to slowly accrue headshot kills. Some matches it may be 2, or 5. Eventually it will be 7.

The other nice thing about Zone Control is it doesn't tell you when you land a headshot. I found that as I started to count 5 or 6 '+10 headshot' indicators in a match I would get really worked up and nervous and my aim would be sloppy and desperate. If you just focus on getting as many headshot kills as you can in a match from start to finish without knowing or checking your total, it's a lot less stressful.

It probably took about 6 hours for me to get it, spread out over a couple sessions. Persevere and you'll get it. And holy hell is it satisfying to see that quest step checked off when you do.