If you haven’t seen the new Tau Lord of War Battlesuit from Forgeworld (i.e. basically the Tau version of a Titan), then you might not be aware of the KX139 Ta’unar’s rules and 120 MB price tag.
Stat-wise, the Ta’unar has S8 T9, a 2+/5++ save (becomes a 4++ against enemy shooting), 10 wounds, and the ability to reduce the number of wounds inflicted by a “Deathblow” result on the Destroyer table by half. Combined with the normal Gargantuan Creature access to Fearless, FNP and ability to reduce poison/sniper attacks to only wounding on 6+ (plus being invulnerable to anything S5 or less due to its T9) and you have the making of an exceptionally reselient “firebase” unit to build your army around.
Firepower-wise, the Ta’unar does not disappoint, with BS4 standard and the following weapon systems:
(I) Two Tri-Axis Ion Cannons — Can shoot either 60″ S7 AP3 Heavy 6 (or) 60″ S9 AP2 Heavy 3
(II) Pulse Ordnance Multi-Driver — Can shoot either 72″ S: D AP2 Ordnance 1, Massive Blast (or) 12 – 120″ Apocalyptic Barrage (3), Pinning, Ignore Cover
(III) Vigilance Defense System — Consists of 2 SMS and 2 TLed Burst Cannons, all of which can fire oeverwatch at BS2 (even through Gargantuan Creatures normally cannot fire Overwatch)
Finally, the Ta’unar can trade out one or both of the its Tri-Axis Ion Cannons for Fusion Eradicators (24″ S8 AP1 Heavy 5, Melta) for free
As you can see, these weapon gives the Ta’unar plenty of flexibility in shooting, giving it the option to engage medium vehicles/MEQs, heavy vehicles/TEQs, MCs, enemy Lords of War, hordes of light troops, and even engage enemy in heavy cover effectively. Being a Gargantuan Creature, it can shoot each of its weapons at a different target, making it even more flexible and capable.
In terms of weaknesses, it is of course expensive points-wise, and it has limited utility in Close Combat (only WS3 and 2A base, as well as the normal Gargantuan Creature attributes of Fear, Hammer of Wrath, Stomp attacks, etc.). Savvy enemy commanders will tie up the Ta’unar in close combat as quickly as possible with cheap, durable troops, thus preventing it from employing its incredible firepower for at least a turn or two.
In terms of killing the Ta’unar outright, with shooting the only realistic solutions are massed Grav Weapons, massed Rending Weapons (the Misfortune power can help with that), D weapons, or Witchfires like Psychic Shriek which exploint the Ta’unar’s Ld9 to inflict multiple wounds at a time. Of these, Grav Cannons with Amps are the mst dangerous (it still takes an average of 45 BS4 Grav Cannon/Amp shots to kill a single suit of Supremacy Armor).
Alternately, if an army has units that can put out a lot of S9/10 AP2 attacks in close combat (i.e. Thunderwolves, Dreadnoughts, Assault Centurions, Monstrous Creatures, etc.), then they can effectively “hammer down” the Ta’unar in the assault phase, provided they can come to grips with the Lord of War. Grey Knights are probably best at this, since they have multiple units that can get both S10 AP2 and Instant Death (from Force weapons), which should rapidly turn the Ta’unar into battlesuit scrap parts.
As for using the Ta’unar in your Tau army, given its extreme durability against shooting, you probably want to “screen” it from assault with cheap close combat troops, like Kroot, or use allied close combat troops for this role (i.e. Necron Flayed Ones or Lychguard would be excellen units to “screen” the Ta’unar from beging assaulted). In terms of target prioroty for its weapons, its versatility lets you really customize its shooting to the threat you are facing. Against most infantry-heavy armies, the Pulse Ordnance Multi-Driver can use its S8 AP3 Apocalyptic Barrage (3), Ignore Cover profile to defeat troops in cover, jinking bikes, etc. If you are facing lots of vehicles, TEQs, or even enemy LOWs, then use the 72″ S: D AP2 Massive Blast profile to throw out 7″ pie plates of doom. The same flexibility goes for its Tri-axis Ion Cannon, which can either be used a 6 shot AP3 Autocannon or a 3 shot Lascannon.
Most importantly, don’t forget that, as a Tau unit, the Ta’unar battlesuit can benefit from Markerlights! That means it can enhance its BS and/or Ignore Cover against priority enemy targets. In a pinch, if you can get enough MLs hits on an enemy flyer, you can even use the Ta’unar’s Tri-axis Ion Cannon in an anti-aircraft role. While the Ta’unar cost (rightfully) about 1/3 of your points in a 1850 game, the key is to support it and prevent the enemy from “marginalizing” it, either by tying it up in combat or preventing it from shooting effectively.