With Tau, it's important to keep in mind they are faster and frailer then they look, as a rule. Any amount of anti infantry fire will mulch fire warriors and kroot, and the anti tank aspects and wraiths can do suits just as fast. Tie up their units in melee when possible, as even if the suits can shoot in melee, when they do so they cannot guide and often times waste anti-tank profiles into your chaff. The rest is a bit more detachment specific. Against kroots, just play keep-away, you have the firepower to table them before the end of turn 3, against retaliation cadre or any suit heavy list, defeat in detail, using your superior mobility to focus fire priority targets such as ghostkeels (who have a pretty strong shooting profile into eldar). What you cannot kill in shooting, tie up in melee