Nightmare actually lets you fight some boss every encounter he sucks you inside, its been a while since I played, but I believe the first time you fight a few lesser demons & then phantom, the second time you fight lesser demons then Griffon, the third time will be Nelo Angelo.
he's pretty much the hardest boss in the entire series. my choice was DMC1.
& about Sanctus, he's really easy, the Sparda Sword shouldn't be avoided, you should just use the DB 1 second before his attack connects, his other attacks are easily dodge-able. Vergil was the most epic battles, either as Nelo or Vergil, though both of them aren't very hard, you can easily avoid their attacks. but they still win in EPICNESS.