Halo: Reach

The achievement description is slightly misleading.

Go into "Matchmaking" and then select "Firefight", under the "Co-Operative" Playlist. You will matched with others and be put into a 4-player game for a 10 minute firefight game. Vote for the Sniper or Rocket gametypes, if possible. Once you get here, its pretty straight forward, basic, and easy. Kill the enemies until you amass 15,000 points AS A TEAM.

Personally, I did it in the Rocket gametype, but from what Ive seen, most people get 15,000 easily in any gametype.

IMPORTANT NOTE: Some people have experienced problems with this achievement unlocking at 16,000 instead of 15,000!

The best way to do that is to go into matchmaking, go to cooperative, score attack, and pick sniper mode. this mode gives all your weapons unlimited ammo if you not good with sniper no problem pick up something easy like the needler it unloads fast can hit your target even if they move and reloads fast. here is my history of the game i played getting it

http://www.bungie.net/Stats/Reach/GameStats.aspx?gameid=8855...

I got this achievement reasonably easily and I'm not even very good at Halo.

Go to Score attack (in the co-op section of the playlists). You want to be playing 'score attack' (or it sounds like 'sniper attack' will also work as StarRoadWarrior mentioned).

Ideally you want to be playing on Courtyard (I found this the easiest). If you don't get given the option of this map, just keep voting 'none of the above' and eventually you will be able to select it.

You should start from the default location in the small room with the ammo store. Go out the top door and on to the walkway that runs along the side of the courtyard (having picked up extra ammo).

For the grunt and jackal waves (1 and 2) I opted for the recon kit - as the DMR will headshot grunts etc. You get extra points for medals - e.g. headshots, double kills, killing sprees. Just be careful of any grunts with fuel rod guns jumping on top of the crates.

Try to bait some of the grunts up on to the walkway and kill them there to give you a supply of plasma grenades (these are a quick and simple way to take down groups of jackals with shields and later brutes).

Third wave is brutes, at this point I lured them back towards the room where you start as you can funnel them through the door making them easy prey for grenades (sticks are especially effective). If you die the Spectre kit is the best to use as the grenade launcher is a one hit kill on most Brutes and they are an easy target coming through the doorway (just be careful of any trying to come through the bottom entrance).

By the end of this wave you should be at or close to the score target. The fourth wave has a bunch of fast-moving enemies so funnelling them into the top room is again quite effective (these guys aren't very bright when it comes to avoiding thrown grenades either). The fifth and final wave has tougher brutes, but if you make it this far you probably beat the required score already.

If you decide to do score attack here is some tips:

Difficulty : Normal
Level: Corvette
Loadout: Your Choice (I used Armor Lock)
Weapons Spawn: Pistol and Shotgun

Go into Matchmaking --> Firefight Once you boot it up it'll normally give you 3 options for what gametype comes up, I did Score Attack.

In Corvette you will spawn on one side in a room with ammo, and 6 medkits, all the enemies will spawn coming out the other door (except for dropships). From the room you spawned in if you go out the right door you will be on a high lip with easy access back to your spawn which is where I would hold up if things got tough.

The waves are as follows:
Grunts
Jackals (Lower Class)
Brutes
Jackals (Higher Class)
Dropship with Elites

For the Grunts Wave I just sat up on the ledge and used my pistol on them, if you aren't the best with headshots I would recommend taking out the fuel rod Grunts and then using the fuel rod to take out the rest.

For the Lower Jackal wave I sat up on the ledge again and used the pistol, I also used grenades at larger groups. These also drop the Needle Rifle which I used when my pistol ran out of ammo.

When the brute wave spawns I suggest some type of headshot weapon, I used the needle rifle and the great thing about that is you don't need to headshot them with it, once you get 3 or 4 needles into a brute it will blow up making it once again easier if your not pulling off the headshots. The brutes did overwhelm me at one point and I retreated in base and used my shotgun to mop up the remaining brutes that came in.

The next wave of jackals are a bit tougher they have the shielding on the arms and seem to resist headshots, I used a mix of grenades and shotguns and stayed within my spawn area letting them come to me. I would save your grenades for at least when there is a group. I also don't recommend being outside as they do spawn with precision weapons and can make it tough on you.

The last wave is Elites via Dropship, be careful of the dropship coming in. I used Plasma grenades and a shotgun on them letting them come to me again. You do not need to kill all Elites to get the achievement, after about 3 - 4 Elite kills you will get the achievement.

Rememeber to be wary of your time left, good luck!

Go to the score attack modes as others have mentioned above. Just look for Score Attack 2x when picking the board and mode. If you don't see it just select the 4 option in the list "none of the above". i wasn't paying attention and got 20,000 by the end of the game. My game was on heroic which may be a default for that mode, not sure. Pretty simple, just survive till the end.