Well, having been rereading vairous bits of Imperium v. Tau stuff - none of which I've found terribly satisfying, have dug up some thoughts posted over on SB RE Imperium v. Tau, with my being as noted rather unsatisfied with G Dubs' handling thereof (and there I go with the understatement again), and consequently each side's tactics against the other. Particularly, the Astra Militarum's tactics, which are going to be focussed on airspace denial/contest and tactical defensive fighting, as well as misdirection. As long as they have air superiority, the Tau have mobility that the Guard can't match, so denying them that, or at least making it much more difficult, is a high priority for IG forces; mainly using air defence artillery (so, Hydras, Manticores, Praetors and the towed-mount equivalents; but primarily the self-propelled models as they can displace easier after firing). Even limiting the proximity to Imperial forces that Tau air transport can operate in would be a major help in retarding their mobility advantage, as well as preventing a direct assault landing. Similar reasoning applies for holding to the tactical defensive; the Tau's edge is in mobility, the Guard's in field engineering and firepower. So use that; try and force the Tau to come into their reach via, effectively, the
hedgehog concept, mutually supporting strongpoints/fire support bases rather than a contiguous line, in order that to reduce each the Tau have to come into the artillery radius of multiple others (as well as doing things like preregistering the best LZs for Battlesuit formations or firing positions for Sky Rays, etc.).
As far as misdirection, well, the Tau Fire Caste are, frankly, a bunch of tactics snobs; they're obsessively wedded to the tactical engagement as the highest art of warfare. And, of the observed Fire Caste senior officers, Farsight's one of the very few able - although O'R'Myr from the Taros campaign seems to be also - to make the conceptual leap needed to see war as something other than a series of disconnected tactical engagements. That gives the Guard options, particularly for using drop troop regiments (or engaging allied Astartes forces for that role) to hit the Tau from unexpected directions and generally upset their tactical planning.
None of this is perfect or supposed to be a magic bullet solution, but it would be a whole lot more interesting to read about than the average (from Phil Kelly, notably; and that git seems to be the only one getting Tau novels), "The Guard troopers file mindlessly into the Tau death-cannons, clogging them with bodies!", which is both ball-achingly boring and doesn't do the Tau any favours either.