This achievement is for obtaining 2,500 headshots in Matchmaking on MCC. However with the release of Halo: Reach a new faster method is now available which I'm going to explain down below with a setup intended for obtaining a few other achievements. Mainly for killing 100 and 1,000 Skirmishers respectively, but this method will also help you grind headshots and double kills.
First, you'll want to head to Firefight in the main menu. Then select Custom Firefight. Using these settings you should be able to setup an ideal solo Firefight for grinding these 100 / 1,000 Skirmisher kills, 2,500 headshots or 250 double kills.
Game Type: Score Attack
-Put Round 1, 2, 3 and Bonus properties to Skirmishers and ONLY Skirmishers.
-Turn off all skulls for all waves including the bonus wave.
-Turn off "Uses Dropship" for all waves EXCEPT for the bonus wave.
-Set the Bonus Wave Duration to 2 minutes.
Wave Limit: Ten Sets
Turn Count: None
Time Limit: I put 60 minutes, but you can put it to whatever you'd like.
Weapon Drops: Enabled
Ammo Crates: Enabled
Spartan Settings > BaseTraits > Shields and Health -
Damage Resistance: Invulnerable
Weapons and Damage-
Infinite Ammo: Enabled
Weapon Pickup: Enabled
You can mess with the movement and sensors if you want to, but it's not necessary. However, leave the damage alone as you'll also be going for progress towards the 2,500 matchmaking headshot achievement.
Now start the game and get to killing. It shouldn't take long at all to hit the 100 kills to earn this achievement. I highly recommend you pickup a Needle Rifle dropped by one of the skirmishers and start going for as many headshots as possible so you can get a ton of progress towards Aim for the Head.
You will also be earning A LOT of double kills during this as well which will make great progress towards:
Very early on within the first couple of waves you should have also popped:
Later down the road after you continue your massacre you will earn: