Successfully complete the game to unlock New Game+ mode.
Successfully complete the game to unlock a Dante's Inferno comic book.
When you gain the ability to charge your holy projectile, Dante is invulnerable while releasing the attack. Use this to evade the closest enemies attack while damaging other enemies with the projectile.
When Virgil appears near a save point, stand by him and press R. He will tell you a part of the story. Do it again, and he will say something else and give you Souls. Save the game, intentionally die, then continue from the save point. Talk to Virgil again to get more Souls. Repeat this as many times as desired.
Jump and press R to instantly kill flying enemies.