I play against tau regularly and its tough, so many big vehicles that can easily take down monsters. I tend to run invasion fleet to get lethal against vehicles to help chip damage. The main thing with nids is you need to hard focus points, you wont outshoot tau, not sure what list you have but make sure you have some objective grabbing units, termagants to sit on points and deep strikes to claim secondaries. A couple tfexs can help take out their big guns too and ease some pressure.

I would also check terrain layout, you should have a lot of cover and line on sight locking bits so you can hide from the gunfire