As I review the new Tau Codex, it appears that the Tau Empire will continue to be one of the top armies, given the buff to current units and new units/formations that they gained. I see them competing quite handily with Eldar, SM Battle Companies, Necrons, etc. as one of the more competitive armies out there.
In order to combat Tau, I think the key is to “de-synch” their synergy, especially with Marker Lights and their new rules that give bonuses for multiple Tau units engaging the same unit(s). One of the best unit, IMHO, to execute this “de-synching” is the Vindicare Assassin.
Reviewing the Vindicare’s stats, he is basically an advanced sniper with BS8 and AP2, Ignore Cover shots, always has Precision Shots (so can always pick the model he is hitting) and adds -2 to enemy Look Out Sir! rolls against his attacks. He also can pick his ammo, either Hellfire Rounds (Wound on 2+, vice normal 4+), Shield Breaker Rounds (Ignores Invul saves), or Turbo-Penetrator Rounds (S10 vs vehicles, D3 wounds inflicted on non-vehicle models for every Unsaved Wound caused).
So, how can these abilities be put to use against the Greater Good? Here are some tactics:
(1) Ethereal Sniping — Using the Turbo-Penetrator Round and the -2 to Look Out Sir! rolls, the Vindicare can reliably Hit, wound, and kill an Ethereal hiding in an enemy squad/in cover in a single shooting phase. This not only removes his Ld10 bubble, but also prevents the Tau player from using Invocation of the Elements to increase the firepower of his Fire Warriors (it also gives a bonus VP for killing the Ethereal).
(2) “Signature Systems” Surgical Strike — Since Crisis Teams Shas’vres can take Signature Systems (like the PENChip and MSS) and buff their squad, thereby freeing up Tau Commanders to focus on other tasks, the Vindicare provides an excellent “antidote” option and, using the Turbo-Penetrator Round, has an excellent chance of “one-shotting” these Shas’vres and their invaluable Signature Systems. This can greatly reduce the efficiency of the unit, especially if it loses Tank Hunter/Monster Hunter, Ignore Cover, and/or Twin-Linked to all of its shooting.
(3) Vehicle Hunter — With a BS8 S10 AP2 shot and 72″ range, the Vindicare can reliably penetrate any vehicle in the Tau Empire arsenal (remember, no Cover saves from Jinking as well). This means stripping a HP, at best destroying the vehicle outright on a lucky 6+ roll, and at worst reducing the vehicle to Snapshots, which can neuter the offensive capacity of Hammerheads, Sky Rays, Flyers, etc.
(4) Squad Leader Eliminator — Using his Hellfire Rounds, the Vindicare can easily remove Firesight Marksmen (reducing the Sniper Drones from BS5 to BS2), Fire Warrior/Pathfinder Shas’ui, Kroot Shapers, Stealth Shas’vre, etc. This not only reduces the Ld of the squad, but also reduces its combat power and also removes any special Wargear, such as MLs, that the squad leader might be carrying.
(5) Battlesuit Team Nemesis — While the Vindicare’s single shot per turn is not enough to threaten entire Battlesuit teams of Crisis, Broadside, Stealth, Riptide, Ghostkeel, etc., there is a way to make him an exceptional threat to these Tau units: Emplaced Gun Turrets! Having the Vindicare man the Krakstorm Missile Launcher on a Fortress of Redemption, the Plasma Obliterator fortification, or the Macro-Cannon on an Aquila Strongpoint turns any of these weapons into a BS8, Cover-Ignoring, Look Out Sir! reducing super Gun Emplacement that can literally destroy entire Battlesuit Teams every single shooting phase. This is not cheap points-wise, but if you build the rest of your army correctly, it can produce devastating results against a Tau army.
(6) Force “De-Multiplier” — While in most cases killing a single model will not be game-changer, sometimes the Vindicare’s ability to surgically remove a key unit can make all the difference. For example, if a Ghostkeel team is down to a single Stealth Drone, it still has an automatic 2+ Cover Save against any shooting greater than 12″ away. However, if the Vindicare picks off that last Drone, then the entire Ghostkeel Team is reduced to a 5+ Cover save, which obviously is a massive difference in its survivability. Similarly, if that Riptide is on its last wound, but has powered up its Nova Reactor for a 2+/3++ defensive posture and has Early Warning Override/Ion Accelerator and is about to Intercept your Drop Podding Centurions next turn, then the Vindicare can use its Shield Breaker Rounds and BS8 to Ignore the Riptide’s Armor, Invul, and any Cover saves and remove that final wound, thus saving your Centurions from eating a S8 AP2 Pie Plate when they arrive from Reserve. Finally, if a Stealth Team Shas’vre is about to bring in an entire Retaliation Cadre Turn 2 with no scatter using a Homing Beacon, you can “plink” that model (and his Homing Beacon) with a Hellfire Round (AP2, Ignore Cover, 2+ to Wound), thus making the arrival of the Tau Formation much less precise (and much more risky).
(7) Marker Light Suppressor — As mentioned, Marker Lights are key to a competitive Tau army. Thus, removing them can be the key to unhinging the Tau battle plan. Whether working solo with his Turbo-Penetrator Rounds, or manning an Emplaced Turret as described above, the Vindicare can potentially kill multiple MLs a turn, even if they are in cover and/or have some models “tanking” for them (the Vindicare can ignore the “tanking” model and shoot the ML models because with his ability to always have Precision Shots). Even if the Vindicare only kills 2-3 ML bearing models a turn, over the course of a game this will quickly add up and, say by turn 3-4, the Tau Army as a whole will have measurably less combat power.