Synapse = Immune to instant death, and fearless.
So pinning, or making them run away doesn't work.
Best option is kill the synapse creatures, and there is only one way to do that. The hard way, by either shooting or assaulting them until they die.
Which gets tricky when it's a Hive Tyrant with 2 guards a T6 with 8 wounds total for the unit typically standing in 4+ cover.
Hand to hand works best IW terminators, etc. Unfortunately for Tau you don't have that option.
Best of luck. I retired my nidzilla list simply because everyone hated it, and after 5 tournaments nothing I fought against gave it much trouble. I'm sure there are specific lists out there that could. However I haven't seen it yet.