This achievement is for killing 100 Skirmishers in Halo: Reach. There is also an achievement for killing 1,000 Skirmishers. This method will explain how to grind out all 1,000 kills while also making progress towards a couple of other achievements.
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 1,000 kills.
Map: Corvette
Game Type: Score Attack
Options -
Wave Properties:
-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.
Scenario Settings-
Hazards: Disabled
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.
Difficulty: Normal
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:
Within the 2nd or 3rd set (probably well before that) you should have your 1,000 Skirmisher kills as well which will also earn you: