Halo 3

Just to clear up some confusion...

UPDATE: Grunt kills do not count toward this achievement!

1. You need 10 headshot KILLS, not 10 headshots. The shot that kills the enemy must be a headshot.

2. The kills do NOT have to be consecutive (10 headshots in a row). Mine certainly weren't.

3. You can do this with any gun, not just the sniper. While I did this without, many folks are saying it must be done with the sniper or beam rifle. I don't know if this has changed, but sounds like it's easier to just go with that :)

4. You can do this in campaign or multiplayer.

Easy method #1: Start the campaign level "The Ark" at mission start on Normal. As you walk up the hill, there are several enemies waiting for you. Kill as many as you can with sniper headshots. If you don't have enough kills by the time all the enemies are dead, simply move to the next area where there are many more. You can take as long as you need and stretch this over the entire level if you like, but by this second checkpoint you probably will have the achievement. Just take your time and aim.

Easy method #2: Play Lone Wolves. You will probably unlock the achievement within a few games, if you can manage 10+ kills (really shouldn't be a problem).

General:
Headshot Honcho is a Halo 3 Achievement that is awarded for scoring ten headshot kills with a Sniper Rifle or a Beam Rifle in a ranked free-for-all Matchmaking playlist or in Campaign. It may be done on any difficulty in Campaign. The Achievement is worth 5 Gamerpoints and is represented by black crosshairs with a sad face in the middle. This achievement is only obtainable through 10 headshots on Brutes or Jackals. Grunts do not count.

Recommendations:

At the start of The Ark, scoring ten headshot kills should be relatively easy, as the Covenant forces are initially unaware of the player's presence.

On Tsavo Highway, at the second outside area (with two Shades), there are several Jackal Snipers. A player can kill one, take its Beam Rifle, and snipe the Jackals or Brutes in the area.

On Sierra 117, a Sniper Rifle can be obtained at the crashed Pelican. It can later be used at the last part of the level, where there are more than enough enemies.

Link: http://halo.wikia.com/wiki/Headshot_Honcho

This achievement can be as easy as _“Insert Food Here”_. The achievement requires you to get 10 headshots in either a ranked matchmaking match or a campaign mission. Not total in all games but in a single game. I recommend trying for this achievement in the first mission Seirra 117 on the easiest difficulty. Once you get so far into the mission you will come across a destroyed falcon/pelican(air transport) and a couple crates near it. Go over to the crates and look around for a sniper rifle. There will be 2 of them there near the weapon crates. Then there will be a couple enemies for you to headshot. Just aim for the head of any enemy you wish and pull the trigger. Repeat this 9 more times(for a total of 10 times) and Bleep Bloop... you got the achievement. There are also Sniper Jackels along the way that you can kill for a Beam Rifle just do the same with the Beam Rifle and you can get the achievement. Hope that this guide helped you raise your gamerscore and get closer to 100%ing this game.
Tips:
For the armored brutes scope in and then aim in the middle of their helmet towards the fleshy part of their face.

Play first level Sierra 117 until you see the crashed aircraft near it there is a sniper (I have tried other weapons they didn't work) keep it until you get too the dock like area and snipe 10 enemies in the head if you miss jump off the edge and it will restart on the top of the cliff it's an easy achievement.

I experienced a glitch where, despite playing a ton of Lone Wolves back in the day and beating the campaign multiple times, I never got Headshot Honcho. Even following guides on here did not work. I think stats were broken and the achievement wasn't being triggered because the game thought I'd already earned it.

Ultimately, I needed to start over on a fresh save to get this one. I couldn't figure out how to wipe my save data completely; even after deleting the whole of Halo 3 from my hard drive, moving my cloud save to a USB and removing it, and then deleting the cloud save from the cloud, the game still loaded my Spartan and my campaign progress-- and thus my stats.

0. I had to use a whole other Xbox to get this to work but if you can find a way to stop the game from loading your stats, this will work as well. You know the game hasn't loaded your stats if your Spartan colors/customization don't load.

1. Start Sierra 117 on Easy and play through until after the Pelican crash.

2. When you reunite with the Marines from Johnson's Pelican, go down to the shoreline and find the crash site. There will be two sniper rifles there. Take one and walk over the other to grab ammo. You should have 24 shots: 4 in the gun and 20 in reserve.

3. Approach the final area and start poppin' heads. Doesn't matter who you shoot as long as it's a headshot.

Others report that you need to shoot Jackals and Brutes, and that Grunts don't count, but this is provably false. See the below video where I unlock the achievement by killing a grunt.