I just beat the 1st boss of the DLC for the second time ever, which I had so much trouble with on my first Faith character. This really taught me one thing: People fighting Sanctuary Guardian who do not have a non-lightning pure damage source are fucked. When I first fought him, I had an occult weapon, chaos weapon, a fire weapon, 2 different divine weapons and the spell Lightning Spear and this fight took me maybe over a dozen times to beat. I was simply stuck. My best option was around 120 damage with Crescent Axe every time he missed a charge or that wind attack, it just took forever.
Now, I have a pyromancer, and even though my only weapon was a Lightning Claymore +5, I still had my pyromancy spells that just ruined him each time he had an opening. I died once, and even that was because I tried to get his tail (and succeeded) and only had 5 estus flasks.
Split damage users are just screwed against that boss, unless we're talking resin or magical buffs