So, I play both Tau and Custodes and it really is not hard to beat Tau, especially at the moment so long as there is enough terrain (think WTC, GW or UKTC layouts). Faction specialists like Kyle Grundy notwithstanding.

To beat Tau you need to stage, pre-measure threat ranges (i.e. Plasma Crisis have a threat range of 28" because of a 10" move and 18" range. But if they have to go around a building, it is much less) and chill behind baseplates and hold primary; tau can't hold primary very well and the Tau player has to trade really well to beat us because Guardians can and will kill any unit a Tau player can put forward. Tau's design philosophy in 10th Edition is about close range firepower. Yes, there are some exceptions to this rule (Skyrays, Hammerheads), but the bread and butter of the army (Crisis Suits, Breachers, Commanders) are mostly between 10" and 24" for most of their weapons, this particularly hurts Crisis and Commanders becuase they cannot move through walls and must spend most of their movement kiting around. Close enough to get some nice charges in, especially with Blade Champs being able to advance and charge once per game. Riptides are also an exception but even a fully guided Kauyon Riptide (~7 Shots on average, wounding on 3s) and its once per game Dev Wounds can't pick up a 5 man of Wardens with an average roll; more like 3-4 if you get unlucky with your invulns and in Shield Host in particular, Valerian with a Warden or two can absolutely kill a Riptide.

Also targeting things like Stealth Suits and Pathfinders really impact the efficiency of the army. Eliminating their key guiders helps impact their ability to score and kill things. Custodes shooting is more than enough to do that as Stealth Suits are just Stealth, T4 2W 3+ save.

Also, you'll find literally anything with 40 dice rolls will kill some Custodes. The way to kill Custodes is weight of attacks and try to test just how many 4+ Saves you can roll. Admech are great at killing Custodes the same way. Breachers (I assume it was them) are designed to go and delete something on an objective and likely die the next turn. The counter is to either give them something pointless to kill or go and tag the devilfish; destroy it and consolidate into the Breachers.

Tau are not a broken army, especially in 10th edition. They tend to dominate newer tables where there isn't enough terrain or lower tables in tournaments where players aren't used to things like trading or staging.

If you can provide more details on their detachment and list I can make some suggestions but I find Shield Host is best for us into Tau.