It has been a while since I did a post on killing Tau, so here goes:
In general, Tau models are relatively fragile and weak. There are many easy ways to kill them. However, they also pump out large amounts of long range firepower, meaning that you will have to survive if you want to kill them. There are several units I have used to combat this:
Deffkoptas: I LOVE Deffkoptas. They won me several games in a tournament at Tabletop Game and Hobby, and made several others very close. One army I played happened to be tau, and these guys were my shining stars. They blew up his HQ (instant death and penetration of armor,) and caused huge amounts of damage on his tanks, saving me from those deadly railgun submunitions. Of course, four deffkoptas cost me a pretty penny, (210 pretty pennies, actually,) but they saved me the game, because before they were blown up, I killed a tank, made another tank completely worthless, (all it had was it’s burst cannons,) and blew up his HQ. But against troops, these guys are a little overkill. (And bigbomms/big shootas don’t penetrate armor,)
Lootas: These guys are a nice choice, they penetrate armor, and can in a pinch take down the tanks. Plant them in cover, keep them away from the enemies railguns, and you will be just fine. They have low BS though, and it will take 1.5 kills each to pay for them self, so you need a cheaper alternative.
Stormboyz: These guys make excellent warriors for taking down Tau Fire Warriors. Get them a looted wagon or hi-jack a trukk, and you can assault anywhere from 28″ (Minimum, only if you take looted wagon,) to 45″ (maximum, only if you take a trukk and call a waaagh! and get double sixes.) On average, you will be getting 36″, though that will fluctuate depending on whether or not you call a waaagh!, (assuming turn 2,) what kind of vehicle you take, ETC. You will obviously assault them, and here is what the average result will be:
(assuming both units have 12 men, orks have PK nob, Tau have Sha’sui)
Orks cause 9.8 wounds
Tau cause .6 wounds
Nob causes 2.2 wounds
So, the entire unit should be precisely annihilated at this point, (exactly 12 wounds on average) and you take MAYBE 1 casualty. Another nice thing about the PK nob configuration is that it can take down takes or HQ units relatively easily, meaning these guys can do everything to damage Tau except maybe hold objectives.
There you have it! Three more ways to kill Tau. (Note: I only have tested Deffkoptas in battle. Lootas I have used in other similar cases, and they performed admirably. Stormboyz, on the other hand, I only just bought, so it is completely math hammer on their respect.)
The Power of the Waaagh! 
As a tau player I would really agree with most of this as the Tau way of life is to avoid return fire and assault while still pumping it out at the same time. What really kills us more than anything else is units that are really really fast. Even the weakest unit in the game, when in close combat, can probably kill the tau. So if you can keep us from being able to jet away and hide, you can probably beat us. A common thing people think about tau is that their strength lies in their long range firepower. I don’t agree with this at all as for me, my optinum strength comes from the range 18-24 inches. The range where my tau’riffic jetpacks come into effect. The tau strength is it’s mobility. For example, the tau hammer head has a 72 inch railgun of death, but it can also take a “10 POINT” upgrade allowing it to move up to 12 inches and still fire it’s railgun. My strategy as tau is to break my opponent all at once, I go a few turns setting things up, and assuming my reserves work out, I can usually win the game in one crippling precision strike. My fire warriors rounding corners guns ablazing, my suits deepstriking, my kroot outflanking, it all happens then. This is what I call the 1-2 KO, as it has always thus far one me a game within two turns of working.
That is why deffkoptas are good.They are really really fast, and cause instant death to everyone in your army. Yes, your tanks are great, but deffkoptas with scout can usually shake or stun them, at minimum. That is also why I suggested trukk boyz and stormboyz, for the HUGE assault range. If I assault all of your fire warriors with my stormboyz, they cant come around corners with guns ablazing, and my deffkoptas, once they keep your tanks from causing to much harm around turn 1-2, and a couple of deffkoptas with rokkits or possibly stormboyz can take care of crisis suits. your deepstriking and outflanking will cause some harm, but Warbikes and rokkit deffkoptas can take care of some of them, so the real threat is kroot. And even they cant stop a horde of charging orks. Also, Orks have a K, not a C. I didn’t see that in this particular comment, but for future reference, here are a few rookie mistakes:
Waaagh! has no R in it, and is pronounced ‘Wha-G’
Ork is spelled with a K, this isn’t fantasy battle
Anyways, glad to see you on the green side of the force!