To A rank the castle level with Aspharr my best piece of advice is to use as many orb charms as you can collect and any weapon that may increase your orb charge speed. Other than that at the beginning of the level rather than running forward into the fray turn around and travel all the way up that long downhill road. Don't attack any enemies until you get to the bottom, and make sure you keep telling your guards to defend. This will make the guards and enemies follow you. At the bottom build up the red orb gauge enough to use a power attack, then go nuts with that to get your orb spark filled. From there you can go up and use it anywhere you see a plentiful group of enemies.
I didnt have as much trouble with that mission as I did Inphyy's forest mission for the A rank.
King of Nights is best done by having the deadly armband equipped, this allows you to use orb draining attacks without losing orb charge. Divine armband, tiara and Ageis charm 3 help keep you alive. When you get to the King, kill off the army of the night and let the king kill all your allies. Dwinggvatt is invincible, use him as distraction for the king. Use the Seraph Butterfly attack to attack the king, this should do a great deal of damage and lands you fairly far from the king. Goodluck!