Alrighty, so Tau has been covered, next I'm going to cover a rarer opponent; Dark Eldar!
Dark Eldar may be behind with the times, but they still pose a very decent threat when piloted by an experienced general. You may not see them sweeping tournaments but they do have some very nasty tricks up their sleeves. For one, their army is a lot, and I mean a lot faster than ours. They can easily get to your board edge by turn 2, and as such will be deciding where and when key engagements will occur. Not only are they fast though, they can dump an incredible amount of poisoned shots on your infantry and drill your vehicles with tons of lances in a turn. Their close combat units certainly aren't bad either with most of them striking before ours and being able to take AP2 weapons that strike at initiative(!!!).
Not to mention that Reaver Jetbikes, especially with Cluster Caltrops, will come tearing across the entire board in some deployment types and tearing apart your backline from reserves. You will have no interaction to prevent their Bladevayne attacks from hitting their mark, no matter how much cover you have. You may have 3+ armor saves, but dumping enough wounds into that will make it seem piddly.
Basically, they're extremely fast, hit hard, and can snowball incredibly quickly off small advantages. They will not play fair and have a lot of tricky pieces of wargear and tactics that can make your head spin. They can be at any objective at any time, and thanks to Shadowfields your weapons will have to get that much closer to begin glancing them.
And yet, they have so many weaknesses. Much like the Tau, Dark Eldar are physically inferior to nearly everything in your army, meaning that they will crumble like wet cardboard when shot at. This also includes their vehicles, which have universally low AV (the highest being 12).
Also, these guys are a Snowballing Army. This means that they need to gain early momentum in order to actually close a game out, as they rely on cornering you into bad positions. If you consistently fend off their advances, they will be forced to play a defensive game to survive, and a defensive Dark Eldar is a useless Dark Eldar. They need to get close, which isn't hard for them necessarily, but they may not be able to once they drop in numbers.
Once again, a few Flamers will help greatly here, especially now that their open topped vehicles will be losing precious passengers thanks to 7th Edition rules changes. Also thanks to the changes to Jink, they will be forced to snap-shot if they want any of their transports or lances to survive, which they really do not want to do as their vehicle weapons are the best shooting their army can field. Autocannons will tear them apart with ease; you'll be glancing on 3's more often than not. Oh, and they'll also ID anything T3 that they have. Nice.
Anything that Ignores Cover is going to be a sound choice as well. You like Noise Marines? Dark Eldar sure don't, because your Salvo 3 Cover Ignoring Sonic Blasters will put out enough shots to actually destroy their Venoms. Blastmasters of course are also the bane of their existence, and taking the Icon will make you far more survivable. Oh, and Doom Sirens? Yes please!
They also lack Psykers, and have very limited Psyker defense. Feel free to go hogwild with boons and debuffs. Anything that reduces LD will be a solid choice as Dark Eldar suffer from lower LD while still wanting to get in close range. This also means that winning combat against them will have a much higher chance of sending them running than your average Space Marine.
Finally, they don't have great answers to Flyers. Heldrakes will are a nightmare for them to face if their Jetfighter isn't on the table, and even with it in play you're not in too much danger since an ADL with Quadgun will rip it to shreds.
With all that in mind, you may be inclined to play a very defensive game against them, perhaps by constructing a solid firebase and gunning them down. Do not do this. If you hold your guys back, they will have free reign over the rest of the table, so you need to move forward. Keeping everyone together though is a sound strategy; a unit alone is easy prey. Make your advancing army extremely scary to approach, and if you have a backline unit like Havocs, put them in a strategically insignificant location, or attempt to bait them with an objective in a narrow passageway.
Daemon Princes are also not the safest option against them since they have tons and tons of poisoned weapons, and plenty of shots to get something through even if he's Invisible or Swooping. Fielding him intelligently though will decimate their ranks very quickly, and if you make him a Psyker you can get some very, very nasty options. Be'lakor, as always, will do a lot of work as well as he preys on low LD and can make your key backline units Invisible when the Jetbikers come in to say hi. Also, avoid taking the Mark of Nurgle; poison doesn't care about your Toughness, and neither do Lances. With the amount of shots these guys can put out, it won't last long enough to be worth it.
I've covered quite a bit above, but I'll list off the important stuff.
- They want to corner you and isolate your units, stay together.
- They may be fast, but they're extremely fragile. Take Autocannons, and lots of them!
- High toughness is meaningless, don't waste points. You need them more for more bodies.
- Quantity over quality. You want a lot of cheaper options to make your army more durable.
- Noise Marines, as usual, are awesome.
- Do not panic, they want you to make a bad choice. Make them play your game.
- Once you have the advantage, force them to take risks.
- Be very, very aware of all of the rules their models possess, they are all very tricky.
- Psykers will punish them. Be'lakor says hi!
- Heldrakes are amazing here for popping vehicles open and killing guys inside others.
- Their weapons are not versatile; poisonous weapons can't hurt vehicles and lances are bad at anti-infantry.
- Low LD = happy day for Crimson Slaughter and Be'lakor!
They are a very stressful army to face at first, but with a little practice you can really start wiping the floor with them. We have a lot of good options against them, just play smart and keep your cool.
- Havocs, 4 Autocannons, 2 Extra Bodies: Taking 3 of these squads and slapping them in cover or behind the ADL will make things very, very scary for them very, very quickly.
- Noise Marines, Rhino, Blastmaster(s), Sonic Blasters, Doom Siren, Icon: In lore and in game, their worst nightmare! Pop vehicles, then melt troops.
- Be'lakor: Yup, still amazing. Be smart with him though, he's going to be a big target.
- Nurgle Obliterators: Yes, I know I said quanitity over quality, but if you don't like Havocs, these are a very nice alternative. Heavy Flamers eat Dark Eldar alive, and can even hurt their vehicles! Plus Assault Cannons, Plasma, Melta...
- A fast unit with the Burning Brand: Biker Lord or Daemon Prince, this thing will do some work. A Slaanesh Biker Lord will also give you Noise Marines as troops!
- Bikers, Mark of Slaanesh, Icon, 2x Melta or Plasma or Flamer, Meltabombs: Really, these can fill any role you need. Flamers for troops (inside or outside of transports), Meltas for transports (glancing on 2's, even though you won't have the Melta rule thanks to Night Shields). Also assaulting a vehicle will typically blow it up through sheer volume of Krak grenades.
- Raptors, kitted out like the Bikers: In case it's your style, helpful for fielding more bodies.
- Heldrake, Baleflamer or HAC: Really both are fine here, though the Baleflamer is usually the better choice.
- ADL, Quadgun: MOAR Autocannons! Also kills their flyer and can intercept the Jetbikers.
- Soul Grinder, Phlegm or Flamer, Nurgle: Combine with TzHerald for Divination powers, blast away vehicle and infantry alike!
- Daemon Prince, Wings, Armor, Nurgle, Black Mace: You know what to do with this guy, just don't get too crazy.
And that's about all I got. These guys are getting an update soon, but if rumors are to be believed (with a grain of salt), most of this should still apply.