Well, rely on the firepower of the Tau if they are your allies, and work to slow the opposition down. Tell your Tau-buddie to do likewise; focus on stationary firepower whenever possible. Then you use Mimes, Wraithlord and Venoms to slow the enemy advance while the Tau firepower takes them apart. Pick on smaller faster untis (assault marines, bikers, trukk boys) and take them apart with your speed and kisses (who are ideal for these aremis for different reasons).
Good luck.
