Divine weapons are used on Undead and the pawns of necromancers (skeletons) and to get a divine weapon, you first need to ascend a +5 normal weapon to a Divine weapon.
Divine Ember: Found on a dead blacksmith past the Moonlight Butterfly boss battle in the Darkroot Gardens.
Large Divine Ember: Found on a dead blacksmith in the Tomb of the Giants. See this video for the location, as describing it in text will only confuse you.
Once you have these embers, take them to Andre of Astora in the Undead Parish. Now you will need the materials. In the end you need:
9 Titanite Shards - Purchased from Andre of Astora for 800 souls each.
9 Green Titanite Shards - Purchased from the Giant Blacksmith in Anor Londo.
8 White Titanite Chunks - Can be farmed by the Bone Towers or the Pinwheel Servants in the Tomb of the Giants, or the axe wielding Black Knight in the Kiln of the First Flame for a guaranteed drop.
1 White Titanite Slab - See 'Occult Weapon' for how to get this.
Once you get a +10 divine weapon, the achievement will unlock.