You need to get any Lightning weapon to +5 for this achievement. To save resources, I recommend doing the Lightning Spear (dropped by mimic in Sen's Fortress) for this. You need 7 titanite chunks and 1 titanite slab for this. titanite chunks can be farmed by the Giant Sentinels in bright Anor Londo or the darkwraiths in New Londo. Titanite Slabs can be easily obtained by completing Siegmeyer's storyline (guide can be found here) and one is a guaranteed drop by the Stray Demon boss on your return visit to the Undead Asylum.