If you're looking for a website for information on optimizing your character for PvE situations, I recommend
http://www.elitistjerks.com/.
They've been around for ages and their forums are the go-to place for character optimization, mechanics, and et cetera - all of it supported by very complicated mathematics.
On the topic of the Icecrown quest achievement, I do believe you'll have to do a decent bit of the group quests there for it, as I found it impossible without blowing through most of them. The Unholy Rider quest can be tricky at first, but I was able to solo him
as melee just by kiting him around and avoiding the zombies (which move rather slowly and despawn after a short period of time). A Warlock commented on the quest (
http://www.wowhead.com/?quest=13161) on how he did it using his demonic circle while kiting and avoiding zombies.
The followup quest after killing all 3 Riders is phased and others can't help you with it, though with the 15% movement speed rune from the Bone Witch, I believe he's kitable (the guy even moves slower when he hits 20% as well).