With Tau & the Farsight Bomb the biggest thing is tactics

Winning the roll off for 1st turn is essential as you want to come in 2nd

Deploy teaser units completely behind cover & don't even bother coming out to shoot

If you end up going 1st wipe out his 800 points on the board before his bomb comes in

Whirlwinds/thunderfire cannons etc at opposite ends of the map so he has to come deal with them and reserving everything else if going 2nd and pack a lot of big guns as they come out of reserve

Psychic powers (invisibility) + others Tau can't do squat against psykers he'll not come any where near an invisible unit

Get yourself a save of some sort DA Librarian with PFG DA Grand Master with 4+ invuln for unit Psychic 4+ FNP etc etc etc

You've gotta remember that the Bomb costs a fortune the Broadsides your always getting a save from unless you take 4+ armor and for the army to have ignores cover their pathfinders have to live beyond the 1st turn