Sanctus looked like a Final Fantasy boss to me. Once a fragile looking bishop suddenly turned into a big monster, or rather, merged into one. It reminds me of the many forms of Seymour in Final Fantasy X.
It's not how I imagined a Devil May Cry boss to look like. I applaud Capcom for trying something new and different, but it just didn't felt as epic as when fighting Mundus, Argosax or Arkham who were these big monstrous beings that looked like demonic entities. In the end, Sanctus was just a human who couldn't handle the power given to him. Even Arius who was also human turned into this awesome big monster in the Lucia storyline of Devil May Cry 2 and he is considered one of the weaker designs of the whole series. I liked that you had to fight that huge statue of Sparda, but it just didn't feel like an end boss battle.