How do I actually pull off the legendary breacherfish combo?

Need some advice because I feel like I'm doing something wrong, I've only played a handful of games with my Tau and every time my breachers have met an untimely end before doing any damage.

First one was against tyranids and I got within a safe distance to shoot but still be a tricky charge and they used the stratagem that stopped them from being shot at within 9 inches (I think) so they couldn't shoot and then next turn got decimated.

Next up was necrons which I'll be facing again tomorrow where my friend runs blobs of 20 necron warriors and they just sit on objectives and I get over watched too hell the second they get out.

Just feel like I'm missing something so any help would be appreciated.