You need to upgrade a raw weapon to +5 for this achievement. You need to have the Large Ember (kill Capra Demon boss for key to Depths, enter the depths and the chest next to the butcher's table in the first room to get the ember). After that, take it to Andre of Astora and use it to ascend a +5 weapon on the standard path. You need 7 titanite chunks and 1 slab. The best places to farm titanite chunks are the darkwraiths in New Londo and the giant sentinels in LIGHT Anor Londo. The easiest places to get Titanite Slabs are the Stray Demon boss on the return visit to the Undead Asylum. The second place is to complete Siegmeyer of Catarina's questline. See HEREfor a detailed guide explaining how to do Siegmeyer's story.