Well, first off since the challenge forces you to be either a Drow or Revenant (Drow), you will definitely want to be a Revenant. Below 0 healing is the most efficient healing, and Revenants are able to activate healing themselves while below 0 and then take the rest of their turn as normal since they are no longer below 0 or dazed.
Archer Ranger utilizing Twin Strike is a solid choice for this Lair Assault. I am quite partial to hybrid Ranger|Cleric for most Lair Assaults due to the amount of synergy the two classes have with each other. Ranger has Twin Strike and the daily stances (Spitting Cobra Stance at 5, Skirmishing Stance at 1) for dishing out high consistent damage. Cleric brings a lot of extra survivability and damage buffing from Battle Cleric's Lore (+2 AC just for hybriding), Weapon of the Gods at 5, and Divine Favor at 6.
Being able to spend surges on demand is a good idea and there are several options for doing so. Ranger of course has the level 2 utility Invigorating Stride that allows you to Second Wind on a move action. Cleric gives you one Healing Word, along with your level 6 utility where there are a lot of options for healing. Some items you might take to complement that are Pelor's Sun Blessing (level 3 divine boon) to spend a surge as a minor, and Silver Hart Clasp +2 (level 9 neck) for another spend surge as a minor.
As you've gone through the first encounter of the challenge already, I suggest taking a look at the Feyslaughter weapon enchantment. Perfect for shutting down that apprentice with a prepared action. Flower of the Dawn is a cheap 25gp item which can be used to awaken a person from any sort of sleep. Frozen Whetstone and Bracers of Archery are the easiest boosts to Twin Strike damage you will ever find. Freezing Arrow +1 at 30gp is an extremely cheap way to turn Twin Strike into a controller power, while adding more damage. With the amount of gold available at level 8 you can easily pick up 30-40 of them to use with every attack. Combine with the feat World Serpent's Grasp, and any enemy you hit twice will start its turn Slowed and Prone.
