"Competitive" can mean several different things.
Are you looking to smash face at your local club?
There's no point giving advice on Dark Eldar specific issues since the whole picture will change in two weeks time, so here's a shopping list of what you need to be able to counter (or at least survive without getting tabled):
- 2 Wraithknights + Serpents
- 4 Riptides
- Imperial Knights, singly or in Multiples
- 5+ Serpents
- 5+ Pod Armies throwing 15 or more scoring units over the table
- Flying MCs (melee versions took a nerf, but Psychic/Shooty ones are still plenty powerful)
- Airforce themed armies
- Orks? Maybe? Does anyone have info on where they're at, competitively? I've not seen them used.
Now you don't need to be able to easily defeat all of the above - the lists are so diverse that you can't possibly have an easy time against all of them using a single army design. However there are three main themes:
- Vehicle spam
- MC spam
- Scoring unit spam
Have a plan against those three archetypes, even if your plan against MCs has to be "run away/speedbump" in order to build a list that can viably take on vehicles or unit hordes, for example.
As far as I know (and I could be wrong) the most competitive "pure" Dark Eldar build - i.e. not just allying Baron into a Craftworld list - is minimum squads of Wyches with Haywires in Venoms with Tri-Ravager/Flyer support and pretty much whatever is cheapest for the HQ - sprinkle in Reaver Jetbikes w/ Heat Lances if you have points spare. If anyone knows of a better unallied DE build then feel free to correct me.
To respond to
then he's right - rock/paper/scissors is fairly prevalent in 7th simply because it's easier in the current game rules to dominate two out of three games, and lose/draw in the third, than it is to do moderately well in all three games, for example. However DE are pretty awful at this as they don't scale very well at all once the points go above, say, 1500. You can fit your Venom spam into that easily enough, but then you start having to fill your points with suboptimal units just to make up the numbers, since you're limited (most of the time) to 6 Troops and 3 HS slots.
Of course you could ally, but if you're going to do that then you're better off simply using Craftworld Eldar with three DE units added to them.