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Ok, my friend has tau. In all honesty they are the race i find the most annoying (especially since my friend does awesome with them) and i need some tactics. I played him recently and i lost most things, he lost very little. THE SHOOTING PHASE NEVER ENDS D:
Dark Lances should make Tau cry. Every wound that goes through (which should be most, since he gets at best some cover) will annihilate a battle suit, and the Lances should also make short work of any tanks he's got. I'd recommend Ravagers to deliver the Lances, since they are so very mobile, come cheap and have three of those most delightful weapons. Trueborn sniper squad might also work, but since I've never played one I can't say for sure. Lastly... Reavers. Arm them with Blasters and zoom across the board. If you can get some true-LoS blocking terrain between them and the enemy even after turboboosting, you should be golden (due to the markerlights). Heck, you could take both Ravagers AND Reavers; that should force him into making some tough decisions when it comes to target selection.
Once you cut down his Broadsides and any long-range tanks, the rest should be a walk in the park. Dark Eldar have crazy mobility, and that is your best friend against just about any given army.
What kind of list are you running and at what point value? If you favor Raider Spam, Tau are a very easy army to beat. Make sure you play with the proper amount of Terrain and LoS blocking Terrain, some Tau players will encourage a more "open" field of play.
Simple jump from cover to cover and know what your important units are. One Raider turned sideways protects two Raiders, sacrifice your Warriors to get your Wyches, Bloodbrides, Incubi etc. stuck in. All you need is to survive until Turn 2 because the army is so fast and once you're stuck in, it's basically over. Honestly I would kill the Fire Warriors and Suits first, get rid of all the anti-Infantry. DE Transports are meant to die, make sure you put those Lances to use after dropping off your assault squads so he shoots at them. Once your Infantry is safe, the Broadsides are easy pickings. If he has more of a Mech build, just open his vital transports while Shaking/Stunning the Hammerheads. Sub-Munitions can be a big deal so make sure they don't get used until you can silence them later.
Markerlights aren't that bad if you can block some LoS, remember Markerlights are rarely mobile. A Raider protecting two raiders will take 3 tokens to remove the cover and probably a 4th token to improve BS. They just wasted a unit taking out a unit you offered as sacrifice, so who cares? Those Markerlights are doing nothing and he has less real guns because of it, you come out ahead.
Another nasty trick if you're using the right HQ is taking a Webway Portal. Even if you don't put anything in Reserve for the Portal, it has a psychological effect on gunline armies and will affect how he deploys and reacts to the unit/s holding Portals. Feel free to use it if you think it will aid you in the game, but the threat of it alone is worth the points during pre-game.
Flicker fields, as they are a vehicle invulnerable save and not a cover, the marker lights wont strip you of your save. Splinter canons do a nice number on crysis suits and pathfinders. I find if I can get rid of marker lights, tau become more like IG with weak BS. Smart missles are a pain but fish with BC's and SMS are at a slight disadvantage because they have 24/18 range. Makes night shields a bit better on your SC venom. Dark lances are also pretty good.
Basically, I try to hit one flank. My venoms target the pathfinders and I try to get wyches into CC with suits to throw a panic into the force. Rails will hurt as will smart missles. Use LOS to minimize the rail fire. I run with the duke so Im not opposed to DSing a trueborn venom unit. Its one time I dont mind trading a blaster squad for a rail head. Haywires on wyches are also worth a look in this case. Its not as hard to hit skimmers in 5th with grenades.
A tau flying circus isn't as nasty as it was in 4th, but it's still a tough force if that's what he is using. I'm going to disagree a little with what was mentioned before hand. The tau players I play are good. They take disruption pods which can really negate DL's. And they use marker lights to strip cover and to up BS. Mech tau is never easy to face for DE.
It it worth DSing a unit of 5 scourges/2 haywires to deal with the disruption pod hammerheads?
Tau can really be the scissors to the DE paper. This can be a tough battle no matter which way you look at it. On average almost everything shooty can drop any transport you have. S5 firewarriors @ 30" etc... S7 MPs at 36"... Rails... in a similar fashion to fighting mechdar.
DL heavy lists can work with Nightshields and Flickerfields. Target suits, and reail heads first. Night shields can effectively force mistakes with his MP suits and keep his firewarriors out of range. Webway lists have a place. The Talos with a Haywire blaster will draw and survive may many rail shots if it has cover, especially if he appears mid table.
My best advice is to go first, and shot every DL you have at anything he has cappable of dropping one of your raiders or ravagers. Vect is rather nice for this tactic.
I wouldn't sacrifice Scourges to maybe take out one vehicle. Unless their dice are actually on fire, they will fail a 4+ eventually. Tau are an overpriced book right now, so they can't spam vehicles out.
Scourges & taloi w/haywire blasters for tanks, w/5 or 6 darklight to blow up transports, outflanking mandrakes for gunlines, followed up by hellions, wyches & beasts coming out of a webway works wonders for me.
Shield drones tend to be a pain, I tend to use venom splinter cannons to take care of these pests (also works wonders against pathfinders (Argh! (triple bracketing hurts my brain)I hate markerlights)). Anyone got any other suggestions on taking out shield drones from range?
Fetch me another plaything. This one seems to have broken- Urien Rakarth
I've not tried the new codex against the Tau but is there a reason an assault heavy force (With haywire grenades where possible) on raiders with enhanced aethersails wouldn't work?