In a word “SHOOTING”. There are other things like improved LD and stubborn from an ethereal but shooting is their main thing. The tau shooting is amazing, and this is down to a few main reasons. Rate of fire, over watch, marker lights, ignore cover and skyfire. In a Tau army it is reasonable to expect they can put out 100+ shots in a turn (pushing 200 in short range with an ethereal in some lists), with bonus to hit and no cover this is very scary. Combine this with the fact they can often effectively gain an extra shooting phase when you charge and you are in trouble. So how can I play against this sort of smash list...?
So how do we counter this...?
2+ and 3+ saves.
I think this is a bit obvious but I think people forget it, with a 2+ save you will only die 1 in 6 and a 3+ 1 in 3 wounds most Tau shooting. A lot of the current Meta lists run big units with T3 and 5+ saves and rely on sitting behind a wall. Bring T4 3+ and things start looking a bit better.
Tau have a good range but don’t just run at them as they are much more effective in that short range. Think of a static Tau list it doesn’t want to move until it has to. Also those 200 shots get halved at 22"+. Use the rage on your heavy weapons to punch the static Broadsides, lascannons or lance weapons are very good for this.
Now ironically Tau with all their shooting are not great at killing av 14. One of the best ways of keeping your units alive is for it to be in an av 14 tank or building. All armies have access to a bastion and at 70 pts and with a capacity of 20 (not including the roof seems) a bargain. It can also be used to give protection to those weapons you want to kill Broadsides with.
Kill the Ethereal.
This guy is the linchpin to a static tau army giving stubborn and a LD bonus. He is also worth an extra VP this could help you claw back a game. Note that you need to kill him and his unit in 1 turn so he can’t jump ship. Barrage weapons can be useful for this.
Play the mission.
Objective missions have never been great for the Tau. There troops are great at shooting but are glass cannons and are not very manoeuvrable. Try to kill these units early game, with the Tau players spending a lot of points on the shiny killy stuff there may not be a lot of these guys around (if the troops are dead you have a good chance of walking away with a draw at least).Position objectives as far apart as possible to try and isolate these units, or at least split up the fire base, this could even allow you to make charges you couldn’t attempt before.
Don’t panic when stuff dies.
Tau are a smash list and you have to expect stuff to die, you even have to expect to look like you are losing for the first few turns but don’t despair just take some of them with you and win on mission.
This is just my thoughts
Let me know if you have any others..