You have to defeat the Four Kings for this achievement. They are located in the New Londo Ruins and you need to meet the following requirements to challenge them:
- Defeat the Great Grey Wolf Sif
- Equip the Covenant of Artorias ring you obtained from defeating Sif
- Have the key to access the lever that drains the New Londo Ruins. There are two ways to obtain this:
1. Acquire the Lordvessel and talk to Ingward, the crimson robed cleric on top of the church-like building in the New Londo Ruins, or
2. Kill Ingward and he will drop the key. I suggest you buy everything he has before you do this. This step can be done before you acquire the Lordvessel in Anor Londo.
Once you have all these, just traverse the ruins until you find a narrow walkway leading to a fog wall. Make sure to equip the Covenant of Artorias ring before you fall down the bottomless pit to access the Abyss. Once you fall, the Four Kings will appear soon afterwards.
For an effective strategy, buy the Iron Flesh pyromancy from Laurentius (the pyromancer you saved in the depths), equip armour with high defenses (the stone armour in the Darkroot Garden is good as well as Havel's armour.) and a good elemental weapon (Lightning Spear from Sen's Fortress) or the "Combustion" and "Great Combustion" pyromancies with a reinforced Pyromancy Flame. As soon as the fight starts, run up to the first king and cast Iron Flesh immediately. Then, it's a matter of casting your pyromancies or poking the king with your spear. Iron Flesh will cut down the damage you take significantly, but remember to heal when necessary. Each king will die once you take off all of it's health, generally this is reflected as about a quarter of the big health bar. Be quick because the kings will appear slowly one by one and it is possible to be ganged up by all four of them at once. NOTE: Even if there's one king left (you've killed three of them) more will still spawn despite the name, so don't let up. Once they're down the achievement will pop.
The following video shows you how to get to the Four Kings and also how to effectively beat them whilst two handing a Lightning Spear+5. Iron Flesh is not used so it is possible without it in case you did not obtain it.