I was thinking recently about how Tau play in general, and what our strategies tend to look like. There are lots of experimental strategies out there, like Commander spam and massive XV8 deep strike armies, but for now I'm considering just conventional Tau tactics.
The Tau army, in my opinion, has four classes of units. Those classes are Gunline, Screening, Fire Support, and Interdiction. I'll go through these four classes, and why they are critical components of our army.
Tau are infamous for being a "gunline" army, and it's infamous for a good reason. We're good at being gunlines, even more so in 8th Edition. Our Gunline is comprised of Strike Teams, Fireblades, and Ethereals. Our Gunline should choose 1-2 objectives, and sit on them. Fireblades and Ethereals provide synergy, but importantly, there should be a very high Fire Warrior to Fireblade ratio. One Fireblade costs as much as five Fire Warriors, and should therefore only be taken if his Volley Fire ability can produce more than 10 shots with Pulse Rifles. Ethereals, especially Aun'va, perform the critical role of freeing our Fire Warriors from dependence on Markerlights by providing them with re-rolls of 1 regardless of the target they shoot at. Our Gunline should also be divisible into different components, ready to 'split' if assault units approach too close.
Tau Gunlines are as flimsy as they are powerful. Besides getting into cover and getting durability buffs from Ethereals, we can't prevent counter-battery. What we can prevent, however, is assaults. To effectively prevent an assault, we need to effectively screen our Gunline units with cheap, disposable units that act as a "speed bump" against enemy charges. For this, we have two options. First, Kroot are the traditional Screening unit. Kroot are cheap and (somewhat) effective in close combat, and can counter-charge enemy assault units. Second, Gun Drones are a more durable Screening unit with effective overwatch firepower. To properly protect a Gunline, we need to practice good defensive geometry to not only slow down charges, but also to prevent deep strike units from landing within our lines. This is quite easy, actually- if there is 18" between two units, deep strikers cannot land between them. Use this to your advantage, and with good placement of Screening units you can prevent both melee charges and deep strike assaults. Whatever it's composition though, your Screening force will melt under enemy fire- dispatch threats quickly before your screen erodes away. It's important to remember, though, that Screening units can be easy Kill Points for the enemy if they're too small. Make sure to bring large groups of Kroot or Drones to prevent easy Kill Points.
3. Fire Support
Of course, Tau armies aren't just Gunlines. In fact, the Gunline should only comprise about 25%-35% of your army's total points cost. Tau Gunlines only put out S5 firepower effectively, so we need Fire Support units to knock out durable high-priority targets and increase our army's accuracy. Markerlights should be embedded throughout the army, and having too many 'dedicated Markerlight' units should be avoided. These units, such as Pathfinder units and Marker Drones, can be destroyed with focus fire very easily. Markerlights on Fire Warrior Shas'ui and Fireblades seem ideal, with 1-2 small squads of Pathfinders. More importantly, though, is anti-vehicle firepower- possibly the weakest point of the 8th Edition Tau army. Heavy Rail Rifle Broadsides, High-Yield Missile Pod Broadsides, and Longstrike seem to be quality Fire Support options. Interestingly enough the regular Hammerhead seems lackluster. However, these units are outclassed in anti-vehicle and anti-monster firepower by deep strikers like Commanders with 4x Fusion Blasters, so make sure not to invest too heavily in big long-range guns: you can get more effective firepower elsewhere.
Interdiction units are probably the most vital in a Tau list. As the enemy marches on our static units, we must be able to deep strike in and alpha strike as hard as possible. We do not have the luxury of 'grinding down' priority targets, so deep strikers such as XV8s and Commanders should drop in close to enemy units and attempt to 'delete' them. Bringing Cyclic Ion Blasters, Plasma Rifles, and of course the all-important Fusion Blasters, our battlesuit teams can vaporize enemy infantry and vehicles, hopefully preventing charges or counter-fire by removing those threats before they can act. If one or two models (or one or two wounds) are left after a battlesuit alpha strike, use your long-range Fire Support to pick off the stragglers. For the Interdiction role, XV8s and Commanders are of course the most powerful units. For XV8s, 3x Plasma Rifle or 3x Cyclic Ion Blaster seem to be the highest-damage loadouts, while Commanders should try 4x versions of those variants or the critical 4x Fusion Blaster loadout. XV8 teams should be protected by groups of 4-6 Tactical Drones. Commanders, on the other hand, can only take a maximum of two Tactical Drones- this is too easy a Kill Point for your enemy. If possible use your XV8s' Drones to shield your Commander, or rely on his character status and hide him behind XV8s. If the Commander must act isolated from the rest of the army, then the two-Drone team becomes necessary though.
Tau are a flimsy army, and our opponents know this. If the enemy is an assault army or not, they will try to gain ground on us, to either assault or to get into rapid-fire range. As such, the Gunline and Screening force should serve as bait, drawing the enemy forward while delivering punishing firepower. Our XV8s and Commanders should deep strike in, enveloping the enemy and picking off their most critical units. If we can destroy key units in an enemy force, our Gunline should (hopefully) be able to take care of the rest. Once the enemy gets too close, abandoning the gunline and splitting up will also be an important tactic. We'll lose firepower if our units stray out of "For the Greater Good" range, but it's a fair price to pay to prevent the enemy from pulling off a devastating multi-charge.
For this tactical doctrine, I feel the important units in our new Index will be Commanders, XV8s, Gun Drones, Fire Warriors, and Longstrike. Besides Pulse firepower, we should focus on Fusion, Plasma, and Ion weapons.
Anyway, those are my thoughts for playing a conventional Tau list in the new world of Eighth Edition! Please let me know what you think, and I look forward to discussing Tau tactics!