My brother plays Tau & they are difficult to crack.
I would go for bikes 10 of them. added T & better save.
Got 1 haemie on a bike & my archon's on a bike as well.
2 ravagers
3 raiders of which 1 with wyches.
Then you could go either way.
Get a haemie & a portal
or
Go for another raider with some more troops.
Another combo which is my actual favorite is the warp beast pack.
Oh & do not forget that our bikes get to move in the assault phase as well which is fun to use vs. Tau
The best tactic is to make him run! Charge the 24" forward & hope for the best (glancing hits). He will not kill everything, you will get something in cc the next round and start killing him off. Note the archon & haemie in the bike squad are just awesome killing cc machines! TAU suck in CC!!
Watch those stupid stealth suits though they will hit you first with an S4 I believe.
I think a portal army would be best against Tau though. The HQ has the ability to aim at different units per model in the unit. They all have targetters if wanted...well not all that good vs. limited amount of targets (they will die)