This is overall an old thread. Way before 1.2 when it was started. But I'll offer this:
Any quest class quest can easily be beaten at level, they're much easier than non-class quests in the same level range, and insanely easier than heroics-2, which can be beaten with 2-3 levels over the quest, a lot less in some cases.
If you experience a bug in which creatures respawn inside a quest (except where it makes sense story wise, but it's fairly obvious since there's usually an animation of them storming the place, but even if the animation were broken or your settings are set to not show animations, following the story also tells you that evil reinforcements are coming) then obviously the quest will be much harder than intended. I have NEVER experienced these bugs, and I'm not sure ithey exist, but whatever, I'll take what people say at face value.
When you let enemies group up too close together, there's a chance your CC will break because your npc ever so briefly looks at the other target while evaluating what to attack next, and they sometimes do choose CC'd targets, although usually they don't. - kind of like players really, heh. Solution, be smart about the CC, don't use them when the enemies are already all on top of each other.
There are many many many tactics you can use to overcome tougher fights. Switch to a healer, NPC, kite enemies around solid objects while you put dots and shoot projects on them, while replenishing your shield as you can. (I've beaten heroics this way after the shtuff hit the fan and I'm left alone with the boss at 25% health... took a while though)
The tank NPC Qyzen rocks, he is really solid. Not as good as a real tank mind you, but with a real tank I can do heroics 4 with 2 people, so I don't need a 'real' tank. Gear him with defence stuff, and defence augments if you haven't. Always start the fight with him shielded but the no-reshield debuff about to expire in a few seconds. Go maximum damage (don't save force) on your first enemy (ideally your only enemy because you CC'd the rest... but I seldomly bother unless they're elites or strongs) then after 6 seconds toss another shield on Qyzen then more bursty goodness until he's ready for another shield then more bursty goodness - it's rare you get to cast a third bubble on him.
Don't be afraid to interrupt your stone throwing to hit a quick interrupt, and don't forget the stun when the enemy is about halfway dead - not because you need it, but so you don't need any healing at the end of the fight, heh. If it's a boss don't be afraid to burn all interrupts including force speeding towards the boss and force wav-ing him away from a big cast.
If not CCing because it's not an option, or you're lazy, weak targets are the priority. heck even if you're CCing, still break the elite/strong as last.
Not sure if this helps anyone, but perhaps it will.