This achievement is for defeating Seathe the Scaleless, which you'll have to do sooner or later in order to progress with the main plot. Towards the end of the game you'll be required to kill Seathe after powering up the lordvessel--after which you'll need to venture into the Duke's Archives from Anor Londo (located up in the hill past the very beginning of Anor Londo--when you first show up there will be a golden fog door which cannot be passed until the lordvessel is placed later on).
Early on in the archives, you'll run into Seathe rather abruptly--yet about twenty seconds into the fight he'll use an attack that is guaranteed to kill you and you won't actually be able to do any lasting damage. Do not lose heart--this is scripted! Scripted or not, you still lose your souls and humanity, so be forewarned you ought to spend / use both prior to your arrival. After Seathe downs you, you'll wake up in the Duke Archives prison which will be the start of the true rest of the area--culminating in finding the Crystal Cave beyond the structure altogether.
Crystal Cave can be pretty daunting at first blush, primarily because you will be required to traverse a minimum of two invisible bridges over killer drops; you can identify the bridges by the slow tinkle of little crystal shards which will break on their surface. The good news is, once you get a general idea of where a bridge is, you can more or less bee-line straight across on both. You'll want to kill the crystal golems here on your first visit at least in order to give yourself comfortable breathing room to assess where the bridges are--but should you die along the way to / at Seathe, I personally found that it's just as easy to run past the guys and book it down the bridges.
Before Seathe's boss chamber, there will be a clambake of crystalline clams; these guys are a renewable drop of Twinkling Titanite and the only place you can truly 'farm' it reliably--but their drop rate is pretty low even with the Covetous Gold Serpent ring, so be forewarned. There is NO bonfire between the Duke's Archives and Seathe himself, but the Crystal Cave is pretty short and, as said, once you've been through once you can just run past all the crystal golems should you need to head down again.
Seathe himself awaits at the very end of the Crystal Cave--and do note that the first time you fight him here there will NOT be a fog door warning you that a fight is ahead. You'll see a glowing crystal at the far end of the room (and probably plenty of cursed statues from other players who died)--and when you approach it, Seathe will come down to fight you. Once the fight starts, you -must- make your way to this crystal and destroy it immediately; until it is destroyed, Seathe will automatically heal back any damage dealt to him and your efforts will be for naught.
Fighting Seathe will either be fairly easy or fairly difficult, depending almost entirely on whether or not you've got solid curse resistance. Nearly every attack Seathe does floods much of the chamber in front of him with crystals which deal heavy curse effect--and if your meter fills up, this can be lethal and end the fight quickly. Apart from this, be very wary of his laser attack which you can see a wind-up for when he rears his head about--as if you are hit by this laser, you will also be killed instantly.
If you get behind Seathe, you can chop off his tail for the Moonlight Greatsword--which is a fantastic weapon if you've got a high intelligence. The tricky part is that you must attack the middle tail, not the side tails, and rather than attacking near the base you want to try to attack towards the very tip--as it will only sever a little over halfway out. When you are behind Seathe, he will start slamming his tails around periodically, which is a fairly heavily damaging attack which also cannot be blocked.
Stay relatively close to Seathe (just not directly adjacent) and you can avoid most of his truly dangerous attacks and get hits in between them. Don't bother with blocking, your shield won't help in the fight for defense so just two-hand your best weapon and hit him as hard as you can. Alternatively, if you have a strong range attack, powerful magic, or the Crystal Ring Shield (formed from a +10 shield and the Moonlight Butterfly soul), you can engage Seathe pretty safely at range as long as you avoid his laser attack--making the fight pretty easy.
I highly recommend bringing good cure resistance when you fight Seathe; the Armor of Favor you can get by completing the Lautrec storyline successfully will give you about 88 curse resist by itself--and in New Londo Ruins you can also acquire the Cursebite Ring, which coupled with the favor armor will have you sitting comfortably around 170 curse resist. Between this and getting used to the tell for his laser attack, you should be able to get the hang of this boss fight and earn this achievement in short order.
After you defeat Seathe, a bonfire will appear inviting you to warp out or start farming crystal clams at your leisure--but you cannot warp -back- to this bonfire, so once you leave you'll have to truck through the archives + crystal cave again to reach it.