Tau are hitting heavy at the moment in the 40k scene. They are a great codex and bring the usual high amount of Tau dakka along with some very cool items of wargear and units to buff them even further to greatness.
A typical competitive Tau list would look something roughly like this:
Tau - 2,000 points
HQ
Commander - command & control note, puretide chip & iridium armour
Ethereal
Elite
Riptide - twin-linked fusion blaster & stimulant injector
Riptide - twin-linked fusion blaster & stimulant injector
3 x Crisis Battlesuits - missile pod, plasma rifle & target lock
Troops
6 x Fire Warriors
6 x Fire Warriors
6 x Fire Warriors
6 x Fire Warriors
20 x Kroot - sniper rounds
10 x Kroot - kroot hound
Fast Attack
8 x Pathfinders
8 x Pathfinders
Heavy Support
2 x Broadsides - high yield missile pods & velocity tracker
2 x Broadsides - high yield missile pods & velocity tracker
2 x Broadsides - high yield missile pods & velocity tracker
Fortification
Aegis Defence Line
Total: 1,993
How it works
The Tau list above is pretty much a static gunline. The bulk of the force will use the ADL for cover along with other bits of scenery i.e ruins. The large Kroot unit will deploy at the front and act as a bubblewrap for the other units. The Ethereal will attach to a Fire Warrior unit and all others deploy around that unit. Pathfinders take a flank each, this way their markerlights can across fire and should reach almost anywhere on the board. To support them a Riptide will take each flank. The Commander will join the Crisis Suits creating a destructive anti transport unit, they can deploy in the middle, but best inside a ruin of something which blocks LOS so they can pop out. The Broadsides would spread along the ADL, they can support the Pathfinders and Riptides and then the Crisis Suit unit. As the Broadsides are in pairs this means if one dies and if the survivor failed a morale test then the remaining model can still regroup as 50% of the unit remains ;) The smaller Kroot unit will outflank (Hound has acute senses so can re-roll which side to come on) and grab an objective mid field.
The Ethereal gives all +1 shot to all pulse weapons, so the Fire Warriors will be getting 3 x S5 shots each at 15". This will effect Kroot too if they use the pulse rounds instead of sniper rounds. In addition all models within 12" use the Ethereal's leadership; this is very handy as Tau love to run away.
Broadsides destroy transport vehicles and flyers, each unit can knock out 8 x S7 shots which are twin-linked and 8 x S5 shots which are also twin-linked, but these shots from the smart missile doesn't need line of sight and also ignores cover.
Crisis Suits will be going for monstrous creatures and armour, the Commander's puretide chip can give monster hunter or tank hunter special rules. In addition the commander and control node allows to re-roll to hit.
Sniper Kroot will be targeting monstrous creatures, as the Kroot wound on a 4+ thanks to sniper rounds, monstrous creatures should go down fast.
The Riptides bring a little bit of melta but really act as fire magnets due to their large size. The nova reactor can be charged to let the burst cannon fire 12 x S6 rending gets hot shots. If the Riptide is under fire the nova reactor can be used to get a 3+ invulnerable or jump away using 3D6".
Weakness of Tau
A typical list like above has little or no melta. Tau's fire power is S7 strongest so will just bounce off av14. If you are using mass av14 such as multiple Land Raiders or Leman Russes, then use them to shield your more vulnerable vehicles and keep them behind.
Of course the lack of melta can be overcome by taking dual fusion blasters on Crisis Suits and deep striking them in. This Fusion Suits will act as suicide units, but they are very cheap and if they take something out then who cares in they die? Plus with them roaming around behind the opponent's force means they will go for linebreaker and also cause a distraction.
While Tau has a lot of dakka it doesn't have mega range. The longest range Tau have is 36", which isn't nothing to be sneezed at, but if you can out range Tau and can camp back and do it, then you will force this mostly static force to come forward. Of course a cunning player would place the ADL more further forward and move up and park the troops behind it or just deploy 12" from the table edge with the ADL.
Speaking of the ADL, it will only give cover to weaponry which is firing directly at the Tau. Ordnance barrage which drops down will seriously ruin Tau's day as they will be bunched up together for the Ethereal bubble and supporting fire. Flamers are particularly nasty, though of course you need to get them close; really it is only the torrent flamers which will work.
Close combat is a big weakness of Tau, most models are weapon skill 2 and have low initiative. Beware of the Crisis Suits though, they are S5, but still have poor weapon skill and initiative. Of course Tau have something to combat close combat and that is supporting fire, this rule allows all Tau units within 6" of a Tau unit who is about to be assaulted, to overwatch. A static gunline and the Ethereal's abilities will mean a Tau force like this will be fairly compacted.
As Tau are mostly foot based it means they are not too awesome at objective games and 40k is all about objectives. To force the Tau to move out place the objectives as far away from them as possible, with Tau spread about and moving out from beyond the ADL this means supporting fire will not work so well and the small units of troops can be isolated and picked on.
Though, this is why Tau lists feature Kroot with Kroot Hound, it allows them to outflank and get objectives. Though Kroot aren't hard to kill.
How to combat Supporting Fire
Supporting fire is a great rule which Tau have, but you can work around it if you use your head. You need to pick your target carefully and use pre-measure to your advantage.
Lets say there are 3 Fire Warriors together in a row. In case of blast templates the Fire Warriors have been spaced out 4-5" apart from each other.
The target of assault should be one of the Fire Warriors on the left or right. If you attack the centre unit you're going to get hit by three lots of overwatch. If you go for the left or right then only the centre unit will be able to help out with supporting fire as each unit is spaced 4-5" apart and then you have the gaps between the models and the models themselves. To combat against support fire even more use pre-measure again and check which models keeping the centre unit in range, all it needs is a single model to be in range and supporting fire will kick in. Shoot the centre unit and deplete the numbers, with the right positioning the models within range for supporting fire will be killed off and this should mean that unit will be out of range for supporting fire.
Kill the Fish People!
As the Tau force is behind the ADL it means practically all of them will be getting cover. If there are good LOS blocking terrain pieces on the board then the Riptides and Crisis Suits can bounce away and hide.
Top target priority when fighting against Tau are Pathfinders and Broadsides.
Pathfinders pimp the Tau by dropping markerlights on your units, these markerlights can take away cover, pump up ballistic skill of the Tau and even call seeker missiles. On average a unit of 8 Pathfinders should hit 4 times, that's enough to take away cover saves and give +2 to the Tau ballistic skill.
Luckily Pathfinders are pussies (basically Guardsmen) and go down pretty quickly. You need volume of fire power to quickly take care of them as Pathfinders will in no doubt be in cover. As they have a 5+ armour save the Pathfinders no not like flamers and other weapons with AP5 or below which ignore cover. As soon as the Pathfinders are gone the Tau won't be so shooty and you won't be losing cover saves.
Broadsides will be opening up your vehicles pretty easily with their volume of fire power and also knocking flyers out the sky as they have skyfire. The Broadsides are pretty damn tough too, 2+ armour save, T4 and 2 wounds. Small arms fire does not work against these unless you have a lot to shoot i.e Ravenwing standard of devastation. Instead you need to bring out the big guns and use things like plasma guns, meltaguns, lascannons, bright/dark lances etc. Anything which is low AP and double the strength of Broadsides will take them down quickly. Don't forget that if a single Broadside remains it can still regroup using normal leadership as 50% of the unit remains.
Once the Broadsides and Pathfinders have been dealt with you will notice your units will start to survive better, though unfortunately you are not out the woods yet.
Target priority will depend on your list; if you have many big monsters then the Sniper Kroot will be target priority as they have the ability to drop a monster per turn. Kroot go down to small arms fire and ignores cover weapons as they only wear loincloths or something. If you want to get into assault then you will have to bypass the Kroot bubblewrap, this means taking out their models and leaving holes in the bubblewrap which your assault units can pass through and launch an assault. If you do fancy assaulting Kroot just keep your mind where Tau units are because of supporting fire.
If you are not running monsters and using vehicles then the Riptides and Crisis Suits should be target priority for you. Riptides will be slightly easier to kill as they are so big it is almost impossible for them to have LOS totally blocked to them. They will get cover though fairly easy, but most times cover will be just as good as their invulnerable save. Riptides do not like high strength low AP fire power, so dakka them with and to be on the safe side have small arms fire units around just to help finish them off. As mentioned above, the Riptide can use the nova reactor to get a 3+ invulnerable save. Like all Tau the Riptide is rubbish in close combat, so you can tie it up which will stop it shooting. Just keep in mind that the Riptide is a monstrous creature so no armour saves for you and it can also use smash attacks. Always remember that a single wound monstrous creature is just as effective as a full wound monstrous creature, but like everything else, the more saves you make it take the more chances of failure, so bring on a fist full of fury.
Crisis Suits are like the Broadsides except they have a 3+ armour save. They will go down to small arms fire reasonably well, though doubling them out with krak missiles, lances or just high strength fire power will do nicely. The more saves you make these guys roll the more chances of failure. If the Commander is at the front tanking the wounds with the 2+ armour save, make sure you fire low AP shots first which will be passed onto other Crisis Suits.
At this point the Crisis Suits and Riptides should be neutralised or at least going down. There is light at the end of the tunnel!
The Ethereal is the next Tau model up for target priority. This fella is quite a buffing machine and needs to die. Depending on what type of army you're using will depend how quickly the Ethereal needs to die. Also the Ethereal becomes more effective the closer Tau get as each Fire Warrior can shoot 3 x S5 shots at 15%. Like the normal infantry, the Ethereal is pretty weak can can be instagibbed by S6 or above. I wouldn't try engage the Ethereal in close combat unless the coast is clear from supporting fire and the Ethereal's unit has been depleted.
Here's a few armies which I think would do ok against a Tau list like the sample one:
Mech Guard - the Guard can out range Tau with some of their guns and their av12 can shrug off most of Tau's fire power. Ordnance such as Manticores they can just drop straight down behind the Tau's ADL and blow them to bits. Advancing Chimeras would shrug off most of Tau's fire power, even mass high yield missile pods would struggle. Once the Guard are close enough double tap plasmas into Riptides with melta going into the Broadsides.
Venom Spam - a Tau list like this would be a struggle for the Dark Eldar, but I think it will come down to whoever can go first. If Dark Eldar do go first then Venoms would splinter Pathfinders to death and any other infantry they can see; they would make short work of the Crisis Suit units. Dark lances would easily instagib the Broadsides while lances and blasters gang up on a Broadside a turn. If Fire Warriors were cleared and isolated then Beastmasters could launch an assault and be in the thick of things. Of course all of this is easier said than done as all of Tau's weapons will easily trash Dark Eldar's light armoured vehicles and destroy their infantry.
Serpent Spam - if Pathfinders were quickly killed by mass scatter lasers and shuri fire, then the Serpents would survive with the awesome holofields. If those Pathfinders remain then the Serpents probably won't be getting any cover at all. Lances, meltas and wraithcannons can deal with Broadsides and Riptides, especially Riptides if the wraithcannons rolled a 6+ to wound - instant death!
Ravenwing - Ravenwing can quickly move up and be in Tau's face, at first glance this is where Tau want them to be so the Fire Warriors can get triple shots, but really it is bad news. Tau like to sit at range and blast things, they do not want to be threatened by close combat. Ravenwing can bring fast mobile melta and plasma which can easily instagib Broadsides and knock wounds off Riptides. They will also have a metric ton of bolter shots which will hose down Fire Warriors and Crisis Suits. Ravenwing can also get into close combat, keep Tau locked in combat in the Tau turn and then hit and run and be ready to shoot and assault again.
Mechrons - Necrons and their mass of av13 vehicles would be a serious problem for Tau, their strongest guns are S7 and would only glance the Necrons' vehicles at the best. Unfortunately for Necrons they lack high strength and low AP weaponry, though elditch lances will tackle Broadsides nicely. Weight of fire will be used to bring units down, but if Fire Warriors have EMP grenades they could easily tackle vehicles which need to get close i.e Ghost Arks.
So far my Tau tactics have been working well and the lists above have done well or I have seen do well.
If you have anything to share about beating Tau face then let it slip in the comment box. Now go smash some blue fishy xenos!