Most Dark Eldar units are Fleet of Foot, so they can get in your face fairly rapidly. They also have extremely high initiative, so charging them is unwise. Despite this, they are mostly S/T 3 and have a 5+ armor on many troops.
Bring out the shooty, and you should do fine. Chaos is decent in close-combat. Plaguemarines with their T5, FNP, and negation of charge attack bonuses would likely make them cry. (Though they wont get heavy weapons)
Also: They have two vehicles that look similar. One is a troop transport. One has three Dark Lances (rocket launchers that treat any armor of 13-14 as AV 12). The best armor value on either of those is 11.
Scourges (DEldar heavy weapons unit, like Chaos Havoc or SM Devastators) have jump packs, so you can potentially have deep striking sillyweapons appear somewhere.
Their Dreadnaught/Wraithlord equivalent is the Talos. It's iffy in shooting, but decent in close combat. S/T7