I've been writing about the monster building process on my blog, Pact of the Tome. Included is an article where I added Fighter levels to a Hill Giant and analyzed CR, as well as building a Battle Mage NPC (basically a level 5 PC Wizard with some of the special features stripped away).
In the Battle Mage article in particular, I found that the fireball spell, combined with an essentially at-will shield and mage armor, made for a quite formidable creature (CR 5) despite hit points falling into a very low range.
In the Battle Mage article in particular, I found that the fireball spell, combined with an essentially at-will shield and mage armor, made for a quite formidable creature (CR 5) despite hit points falling into a very low range.