Shifting from Eldar to Tau, here is my thoughts on how the Deathwatch can handle some of the more dangerous units from the Tau Codex:
 Riptides with Ion Accelerator and Early Warning Override/Stimulant Injectors – One of the classic Tau “power units,” I would say it is rare to find a competitive list without at least one (if not 3-5, as some armies go all out with things like the Riptide Wing formation). The Ion Accelerator provides long range infantry killing power at critical S8 (so able to double out T4 and below) and AP2, while the Early Warning Override makes it a very potent defense against Deep Striking units that would otherwise be a significant threat (e.g. the GHOST EAGLE Battle Group riding in a Drop Pod I described in the counter-Eldar article). With T6, 5W, a 2+/5++ save (which can be improved to a 3++ via the Nova Reactor), plus the option to take a Stimulant Injector for FNP, the Riptide can be a challenge to take down, especially if you can’t use a Drop Pod alpha strike against it because of Early Warning Override. So, how would a Deathwatch army deal with Riptides efficiently? Here is my best take so far —
“PALADIN SPEAR” Battle Group configuration:
-Dominatus Kill Team with two Vanguard Veterans (both with PF/SS) and eight Veterans (4 with Frag Cannons and Bolters, 4 with Plasmas and Bolters), all riding in a Corvus Blackstar with the Auspex Array upgrade and TLed Lascannon/Stormstrike Missiles.
So, I think the key here is to not Deep Strike from Reserve (since the squad might get sucker-punched by a S8 AP2 Interceptor Pie Plate before it even gets to engage). To achieve this, the entire squad starts in the Blackstar, which itself begin the game in Hover mode on a Skyshield Landing Pad (you would have to take an allied CAD to get this, of course). The Blackstar moves 6”, disembarks the PALADIN SPEAR Battle Group 6”, and then the Blackstar and Kill Team shoot at the Riptide at 12” with 8 x S9 AP2 Frag Cannon shots, 8 x S7 AP2 Plasma shots, and 5 Lascannon/Stormstrike Missiles shots by the Blackstar (at BS5 due to Strafing Run). With Dominatus Tactics and Dominatus Doctrine activated, this yields an average of 5-8 Unsaved Wounds after both the Riptides 5++ and FNP saves, which should be sufficient to kill the Riptide outright in that one Shooting Phase. As with the other Battle Groups, the PALADIN SPEAR is specifically designed to kill Elites like the Riptide (or Crises Suits, Stealth Suits, Ghostkeels, etc.), but I think can be effective against other types of units, and even other armies besides Tau, with the Vanguard Vets providing “tanking power” up front via their Storm Shields and Close Combat capacity with their PFs, the Frag Cannons and Plasma Guns allowing engagements against all forms of infantry and medium-light vehicles, and the Corvus Blackstar adding mobility, additional firepower, and a rapid objective seizer as required.
 Pathfinders – One of the “cornerstone” units of the Tau army (if not THE cornerstone unit), Pathfinders are the most efficient source of Marker Lights for most Tau forces and often begin the game camped in heavy cover to maximize their survivability while “lighting up” (literally 😉 ) the enemy army for improved accuracy, Ignore Cover, and/or Strength D Destroyer Missiles. Thus, it goes without saying that removing Pathfinders efficiently is one of the top priorities for a Deathwatch army, so how can that be done? Without spending excessive points, here is my take on how to accomplish this –
“BOWHUNTER ” Battle Group configuration:
-Venator Kill Team with two DW Bikers (both with Power Mauls) and five Veterans (all with Stalker Boltguns and Bolters)
If the Pathfinders are in the open/not in cover, then the BOWHUNTER battlegroup can use Hellfire rounds, which causes an average of 7-9 Unsaved Wounds on the Pathfinders, basically eliminating even a max size squad. Alternately, if the Pathfinders are in cover, then the BOWHUNTER Battle Group can use Dragonfire rounds to ignore that cover, resulting in an average of 6-8 dead Pathfinders, which is still a significant blow to the Markerlight capacity of the Tau army. Since the Bolters and Stalker Boltguns all can shoot out to 24”, the BOWHUNTER Battle Group can likely sit still in terrain in their own DZ and take out multiple Pathfinder Teams (or other Tau infantry, as required) over the course of a game. If the enemy shoots back with AP3/2/1 firepower, then the DW Bikers can tank wounds up front with their 3++ Jink saves and, if for some reason the unit needs to charge, then the two Power Mauls on the bikers provide some anti-vehicle and anti-infantry hitting power. While this Battle Group primarily exists to sit and shoot, it can move and fire when required (thanks to the Veterans having both Stalker Boltguns and normal Bolters) and can deal with a wide variety of non-vehicle targets, thanks to their multi-purpose Special Issue Ammunition.
 Stormsurges – The new “heavy hitter” LOW option for the Tau Empire, this unit can really drop the hammer in shooting with Strength D missiles (with ML support), a multitude of long-range shooting options, and the ability to anchor and “double-tap” shooting. Like with Riptides and Pathfinders, dealing with Stormsurges efficiently is an important ability for any Deathwatch player facing competitive Tau armies. This can be a challenge, given that Stormsurges have T6, a 3+ save, FNP, the “Unstoppable” special rule, the ability to take a 4++ Shield Generator, and 8Ws! I think the key weakness of the Stormsurge is close combat, since they cannot fire Overwatch, only have 2 x WS2 I2 S6 AP2 attacks base, and cannot use Stomp if they are anchored. To exploit this, I think the following is an effective Battle Group for dealing with Stormsurges –
“EAGLE HAMMER ” Battle Group configuration:
-Aquila Kill Team with one Vanguard Vet (TH/SS) and six Veterans (two with SS and DE Shotgun, three with Heavy Thunder Hammers, and one upgraded to a Black Shield with LC/PF), plus an attached DW Chaplain with MB, all riding in a Corvus Blackstar with Infernum Halo-Launcher.
The EAGLE HAMMER Battle Group starts in the Blackstar in Reserve. Turn 2 (hopefully) the Blackstar comes in hot in Zooming Mode and, between forcing Tau units that do not have Velocity Trackers to snap shoot at it and its re-rollable Jink save (i.e. a 75% chance of ignoring Glances/Penetrating hits), survives long enough to position itself for Turn 3, where it drops into Hover mode, moves 6” , disembarks the EAGLE HAMMER Battle Group 6”, and then the Battle Group charges 2D6” (with re-rolls, thanks to Heroic Intervention) into the Stormsurge, which cannot Overwatch because it is a GMC. The three Storm Shields should be able to absorb the Stormsurges two S6 AP2 attacks at I2 and then the Battle Group piles in with 1 x S8 AP1 Melta Bomb attack, 9 x PF attacks and 9 x Heavy Thunder Hammer attacks, all hitting on re-rollable 3+s (thanks to Zealot) and wounding on re-rollable 2+s (thanks to Aquila Doctrine), which results in an average of 16-17 AP2 wounds and 6-7 Unsaved Wounds after the Stormsurge 4++ and FNP saves. While this is not enough to kill the Stormsurge in one turn, the EAGLE HAMMER should be able to finish it off in the next Tau player Assault Phase, thus freeing up the Battle Group at the end of the Tau player’s turn and allowing it charge another enemy unit in the subsequent Deathwatch player’s turn.
Overall, while not perfect by any means, I think these Battle Groups are pretty solid against any Tau opponents and should be particularly effective against these “lynchpin” Tau units described above.