People say not to summon the npc, but my experience is just the opposite. I died like 30 times (most of any boss in any souls game) trying to solo it, and beat it the first time I summoned her. Reason is for the last 10 tries I was using pestilent mist (and offhanding a crossbow). The boss's hp went up, but so did the damage of pestilent mist. And with the npc distracting the boss, I was able to get the pestilent mist well placed without getting hit more often. As a pyromancer with 35 faith, 32 int, I tried the lightning bow spell, but heavy bolts on a +9 heavy crossbow still outdamaged it so, it was pointless.