My advice would be to stick with Tzeentch magic. Easier to cast all of the spells, and you get the spells you want (spells 1,3,4,6) most of the time anyway. But if you are going to use Shadow Magic, most of your spells should focus on protecting your Khorne Marauders. You don't necessarily need the flying carpet. You should have the option to take steed of shadows about 70% of the time. For the spells you get:
Melkoth's Mystifying Miasma to de-buff any unit in combat with your Khorne Marauders.
Steed of Shadows to fly your wizard around.
The Enfeebling Foe to de-buff any unit in combat with your Khorne Marauders or Hell Cannon if the opponent has great weapons.
The Withering is kind of a waste for your army since the lowest strength in your army is a 4, but if you get the spell use it on the unit you want to target with your Hell Cannon.
The Penumbral Pendulum is a waste except against steam tanks and stegadons...maybe. Default it to Miasma if possible.
Pit of Shades is always good. Use it on low initiative characteristic units.
Okkam's Mindrazor is kind of a waste on anything but the Khorne Warriors. Three attacks a piece at strength 8 is nice. Everything else in your army is strength 5, and strength 5 is really good in this edition of warhammer.