This boss encounter isn't terribly difficult--the trick is to avoid letting all the mobs overwhelm you. When you first enter the church-like arena, a bunch of zombies crawl and shamble toward you. You'll notice three slightly larger enemies; two of them shoot difficult-to-avoid lightning bolts, while the third, the Magus him(her?)self, fires slow-moving dark orbs.
Do your best to avoid the projectile attacks while wiping out all the weaker adds, then go for the mages shooting lightning. Finally, take down the Magus. Need extra help? Check out our video above for more strategies.
For more help be sure to check out our complete Dark Souls 2 Boss guide.