One piece of advice: If you can assault Tau, do it. Only in unusual situations would NOT assaulting be a better option (i.e. you can assault and get wiped or you can stand and the firepower won't do the same). If you get locked into combat and its a draw, those railguns can't shoot at you
If you win, you get closer to the broadsides or whatever else you're wanting to eliminate. If you lose...what? Don't lose...and if you do, make sure its "for the greater good"...btw...if you say that often enough, your Tau opponent may get a bit agitated...hehehe
Honestly, I haven't played against broadsides, but my opinion of stuff like that (heavy weapon with great range) is simple...not much you can do about it until you close with the enemy, so make your normal moves and force him to roll the dice. My main opponent uses marker lights every round, lighting up this unit or that unit, firing off his hunter-seeker missile, and hits maybe once out of every 3-4 shots...even if the hit does something, its not worth my changing my strategy any. Just keep wrecking his troop units, making them break, etc...and all else falls into place. Big lesson here: ignore the Big Guns...they can only do so much.