Almost no army can win a shooting war with a tau gunline, and the ones that can are specialized mech lists that struggle against mobile and aggressive opponents. But, Tau gun lines are far from invincible, stay calm and remember these tips.
1. Deployment is now one of most important phases in your up coming game. If you can, make the tau player set up first. Tau gunlines only have two effective deployment options, they can "castle" their army in the most defensible position (usually a large ruin for the 4+ cover save) in their deployment zone. Your opponent will be bunched up in cover to maximize saves, overwatch with friends potential, and to make sure all their units can shoot at the same targets. The other deployment option is a literal line near the edge of the deployment zone, which allows maximum objective grabbing and enemy targeting options. If your opponent chooses to castle your template weapons are now super effective so make sure your list has a few, Ignores cover and long range preferred. If your opponent forms a lose line, deploy your army as close to one table edge as possible. You will be deploying outside of half of the Tau weapon range and will force your opponent to spend a turn or two re-positioning units from the far side to bring them in weapons range. While your opponent re-positions you have the opportunity to use your superior numbers and weight of fire to seriously degrade half your opponents army.
2. Understand the value a long range artillery support unit has against a more or less static force. Lobbas, Thunderfire Cannons, Wyverns, Basilisks, and the daemon warp storm table are all great at racking up wounds each turn and as you degrade the gunline it gets much less dangerous. IG/AM has a special edge with AV14 tanks shooting ID pie plates. Knight Paladins are also fantastic w/ 2x S8 ap3 large blasts and will be brought up more later.
3. Dual or Triple threats are a must. When you can control what your opponent has to shoot at, you are winning the fight. Charging an imperial knight and a land raider full of nasty is the perfect example. Any tau player knows that once they are locked in CC they are flirting with death. Sure they have a few tricks like farsight's 4 ap2 S5 I5 attacks but these are not only eminently manageable, but few and far between. A tau gunline faced with a knight/land raider rush has to kill at least one of those units before the turn 2 charge (no small feet btw). You just freed up two turns for the rest of your army to pound on the tau. Your land raider will almost surely survive till turn two (you should be out of 2d6 melta range turn one) and if it does you can multicharge a few units and will either cut them down after winning combat or stay locked and safe.
4. Learn to love math hammer. Understand the numbers and the rest will follow. The original post said the tau list puts out 60-70 s7 shots a turn sounds like a lot, and it is, unless you bring the right toys. Lets do the math for a Knight on turn one just to show you what i mean. I am going to assume that the front facing of the knight av13 with an active ion sheild 4++ will be hit on turn one. To be as generous to the Tau as possible i'm going to assume the Tau list can put out 70 s7 shots a turn and 35 of them are TL. I'll even be nicer and assume that marker lights have boosted all these shots to bs 4. out of the 35 regular s7 shots 23.333 should hit on average so lets say 23-24. 6's will glance so Tau can expect to average 3-4 glances half of which should be saved on the 4++ shield totaling 1-2 glances on average from 35 s7 shots, OUCH! Now on to the TL shots, 35 TL shots will average 23-24 hits on the first roll, and of the 11-12 average misses 7-8 of those will hit on the TL reroll, totaling 31-32 hits. 32 S7 Hits will average 5 glances against AV13, which will be reduced to 2-3 glances with the 4++ ion shield. At the end of the day this entire Super charged bs 4 tau gunline will only be getting 3-5 glances turn one on a knight. Not even enough to kill it.
5. Know the Tau weak spots. Toughness 4 makes for easy instant death opportunities on expensive and dangerous models like broadsides and crisis suits. Low leadership makes powers like terrify, psy shriek, dominate,special rule like fear great, and creates a vulnerability to LD checks from shooting. Average WS2 and I3 makes Tau prime CC targets and little more then skulls for the skull throne if you have ws 5 and up.
6. Know your armies cheep tricks. If you play CSM, slap a dirge caster on a land raider, multicharge the tau lines turn 2 overwatch free. Play daemons? Be'lakor can invis your important CC units. Cast shrouded your first turn and surround Big B with anything in your army that can jink for 2+ cover save which forces your opponent to use valuable marker light shots or ignore the squad. Play the imperium? Knights, IG/AM armor, centurion stars work great. Etc etc etc...