5x Smite will drop most things on one cast.
5x Lightning Bolt, would almost always require at least another spell.

Priests are horribly mana inefficient, but with five its not so bad.

I went deep enough in Shadow for Mind Flay.
And glyphed, so MF does more damage when SW: Pain is on the target.

You could level as Discipline or as Shadow, either will work fine.
Shadow will be Mindblast, Mindflay, SW: Pain.
Disc will be the same, plus Smite, Holy Fire and at 20+ Holy Nova is very strong.