Comments posted to our Dark Souls 3 Wiki
After running countless damage tests, I have come to the conclusion that this weapon does best with a pure intelligence build despite it's poor scaling after 45 intelligence. Also, it is the better choice in terms of spell buff and spell damage for dark sorceries unless you wish to go past meta levels. Keep in mind I started as a pyromancer, so less than 14 faith wasn't an option (Later testing showed me that the faith stat seems hard capped at 15, offering 1 point of spell buff per point up to 15, after 15 it gives one every other level). But trying different ratios of the standard dark/pyro build of 40 intelligence/40 faith, I got the highest spell buff at 65 intelligence and 15 faith with a spell buff of 225. 60/20 only netted 220. 40/40 got me 202. Alternatively the highest spell buff I could manage with the Izalith staff was 208. At extremely high levels though, the izalith staff will eventually out damage it. Specifically: This weapon at 76 intelligence 15 faith (again, my best ratio in terms damage) got 228 spell buff. The Izalith staff at 60 intelligence 31 faith, also got 228. Ater this though, no matter where I placed subsequent stat points for this weapon, the Izalith staff always out damaged as I brought up faith. At 99 intelligence and faith, the final spell buffs ended with this weapon at 255, and the Izalith getting 289.