Defensive TacticsUse of Cover
With the emphasis on True Line of Sight (TLOS) to determine qualified targets, new commanders will be looking to use cover and movement to reduce the chance of direct enemy attacks against their army's Hammerhead(s). The high profile of the Hammerhead, combined with the fact that line of sight for shooting is drawn from the end of the Railgun or Ion Cannon barrel means that it will often be possible for your Hammerhead to fire over cover immediately adjacent to itself, while gaining a cover save from return shots. This technique is referred to as Line of Sight Sniping or LOS Sniping. This can be combined with the ability of the Smart Missile System to avoid enemy cover saves, and is particularly useful when engaging light infantry. Neither guarantees the complete safety of your tank, but they can extend the life of your Hammerhead.
Note: In the illustration of LOS Sniping below, the Hammerhead depicted is equipped with a Target Lock which, in 5th edition, allowed it to target two separate units. Target Locks are no longer available to Tau vehicles under the 6th edition codex. Without the target lock, the Smart Missile System is required to fire at the same target as the main gun or vice-versa.Heavy Weapon Saturation
Cadres that field multiple Hammerhead tanks take advantage of something often referred to as Heavy Weapon Saturation. An enemy force of a given points value is likely to only be able to field a certain number of anti-tank weapons such as Lascannon, Meltaguns, Dark Lances and so forth. While vehicles are vulnerable to melee assaults as these strike against the rear armour of the tank, if your opponent is shooting at your vehicles, he or she is likely to only have a certain number of heavy weapons capable of actually damaging your tanks. The more tanks you field, the more difficult it becomes for your opponent to destroy them, and this is one of the key tenets of the Mechanised Tau approach, which fields multiple Hammerheads and Devilfish to overwhelm the opponent with durable armoured chassis that many units will be unable to damage.
In simple terms, fielding two Hammerheads makes each more survivable than just fielding a single Hammerhead, and fielding three Hammerheads makes each more durable than if you'd fielded two or one, because there are fewer high-Strength shots likely to be taken per tank. A Hammerhead is usually a high-priority target for the enemy - more so than a Sky Ray, and often more so than a Devilfish in the early stages of a game, so you can expect significant quantities of weapons fire to head their way, and quite possibly attacks from fast-moving assault units capable of damaging the Hammerheads in close combat.
To some extent, the new XV104 - Riptide battlesuit produces a similar effect, and can be used alongside a Hammerhead with similar results to fielding two Hammerheads. The Hammerhead tank vs. Assault troops
The Hammerhead is hampered somewhat by the interaction between secondary weapon systems, defensive upgrades and the rules for attacking vehicles in 7th Edition. While your Hammerhead can move up to 6" and fire one weapon system at full BS and the other as a Snap Shot, the range of the Burst Cannon and the Gun Drones is comparable to that of an enemy assault unit with Fleet
. Broadly speaking, this means that if you're close enough to shoot at those Genestealers or that Wolf Lord on Thunderwolf with the primary and secondary weapon systems, then they're in range to charge you. The Flechette Dischargers can damage units attacking the Hammerhead in melee, but suffers against units with strong saving throws.
While a Hammerhead has a good chance of surviving enemy fire that comes from either the front or side arcs, it remains vulnerable to assault from enemy troops, which will strike against the rear armour rating of the vehicle. Units with Power Fists, Eviscerators, Power Klaws, Melta Bombs and other such weapons can cripple a Hammerhead, and the only defences against this are to keep moving more than 6" a turn - which means forfeiting the tank's shooting for a turn - or relying on the Flechette Discharger to reduce the likelihood of an enemy infantry unit destroying the Hammerhead by stripping its hull points with multiple attacks.
Unfortunately, if you're looking to engage the enemy with the secondary weapon systems, be it Burst Cannon, Gun Drones or even the Smart Missile System at range, you cannot move. A static vehicle is an easy target for infantry, and even glancing hits from Strength 4 models are enough to cause your expensive Hammerhead some concern, as it can only take a maximum of 3 glancing hits before being wrecked. A Flechette Discharger will only kill one in six of every Space Marines engaging the Hammerhead in melee, which means that the bulk of any charging attackers are likely to get the opportunity to attack, and Melta Bombs, Krak Grenades, or even a Space Marine's fist will soon make light work of a Hammerhead.
In summary, when dealing with assault troops, sometimes discretion is the better part of valor. In other words, maximizing the movement of the Hammerhead to reposition it as far away from a potential close combat situation is usually a better defensive measure than trying to neutralize the threat by foregoing the additional movement in order to make use of the secondary weapon systems.
In some cases, your Hammerhead will be forced to Jink in order to avoid damage from shooting attacks. The turn after Jinking is an ideal opportunity to move at cruising speed, then Flat Out in order to reposition your Hammerhead to a safer location.