This is very easily boosted in a 1v1 with another friend. In fact, you can do this, Nemesis, and Hard to Kill all in the same session. This solution assumes you want to do all three at the same time.
Set up a match with all private slots, and invite your friend in. Set the timer to unlimited.
P1(red) sets up as sniper and goes to blu base. Headshot P2. Stay near the spawn spot, and have P2 come out and get his headshot. Do 5 straight headshots (Hard to Kill ach pops at this point).
Before you get the 5th headshot, you'll see that you're dominating P2. After the 5th headshot, switch roles, and let P2 headshot you 5 times, and have P2 move to P1 base. P2 gets a revenge kill for their first kill of you. Let them go forward to get 5 in a row (if they need Hard to Kill ach), and then go back and forth, with each player taking 4 headshots in a row at a time.
By doing it this way, you knock out both the Nemesis ach and the Grey Matter ach. All in, I think we took 30-45 mins tops to knock this out. It's really rather quick to get all of these.