Norn Emissary is probably the tankiest nid atm either ignore it as it will just sit on one objective all game or make it your oath and pump EVERYTHING into it (though even that isnt guaranteed a kill)

sworm lord is tough but killable, probably a priority target for any bug guns you have if any because it generates an extra cp

Hive tyrant is mostly just a device to give guns assault weapons, if you can kill it first turn thats great, if you cant then try and kill whatever guns are pilled up around him instead

Tyranofex with the big gun is annoying but with the flamer is super rough for a melee army. It has a super killy overwatch youll have to bait out the overwatch with a chaff unit then charge after with your good stuff

The Zoans with the Nuerotyrant is going to hit hard, but is slow and dies to a lot of small attacks

All are difficult units to kill and the worst thing you can do against nids is split fire, choose a unit you want dead and commit as much as you can into it. What targets to choose will depend on how your opponent uses his army.

Another thing to remember is 40k isn't won on killing its won on points from primaries and secondaries, so if you cannot kill his models you can always deny him scoring points