It's coming up on 2 years since the release. The game has been torn apart and dissected into the smallest of parts to to fully understand the mechanics of it. The only reason to purchase the guide, or any guide, is for sheer reason of being a collectable item.
Every aspect of the skills, to include the most efficient XP gains, is openly displayed in detail on several sites. Namely, the UESP.
That being said, the only way to increase Destruction is to cast a spell that actually does harm to an enemy. Even then, the XP gains are directly related to the magnitude of the spell and it's magicka cost. The most efficient "bang for the buck" is casting Expert level spells.
On a side note, I see no harm in questioning the intent of some of these topics. Ones that chime in with no purpose but to scold the questionable comments, is more offencive than the harmless prodding of the initiate. In fact, stepping in as a moderator figure is far more arrogant. The fact of the matter is. some of you are just too thin skinned and boast a history of lashing out at anyone that rubs you the least bit.
Difficulty has no bearing on XP gain.