It is well known that destroying the more intelligent creatures in the swarm is essential to stop a Tyranid advance. Training in recognition and fire discipline is of some help in identifying the best targets but the chaos and confusion of the battlefield make it difficult for troops to pick out their targets amidst the swarming mass of creatures. Ultimately it has proven best to direct fire at the largest Tyranids in sight and pray to the Emperor that some of them are the leaders.
First a quick note about tyranids: the big ones usually provide brains to the little ones. Shoot the big ones and it is possible the tyranid player won't be able to control what they do. They have to make a leadership test for each unit that is out of synapse (12" range from my of the big ones). If they fail it is likely they'll either run towards the closest unit or shoot at the closest unit.
A second quick note: the big ones also provide a 12" range bubble that makes it tough for you to do psychic powers (this includes force weapons). You have to roll 3d6 and any doubles results in perils of the warp.
Hive Tyrant: provides synapse. Big monstrous creature, 2d6 pen on vehicles. Power weapons. 3+ save, NO INVULNERABLE! Since he is a monstrous creature, it is tough to get a cover save. Nasty nasty nasty psychic power is "paroxysm". This makes the target unit BS 1 and WS 1.
Raveners: can come in from deep strike. Tons of attacks. Beasts (12" charge, fleet). Often has rending. 3 wounds at Toughness 4! Rapid fire them to death.
Termagants: usually armed with 12" str 4 assault gun at BS3. Cheap. Can be buffed by tervigon.
Zoanthropes: Synapse creature - they have psyker powers, one is a str 10 lance and one is a str 5 ap 3 blast. Both have to roll to hit. Shut them down with librarians.
Hive Guard: Toughness 6, 2 wounds, they shoot a 24" assault 2 strength 8 shot great for taking out transports. Don't need LOS. Priority target.
Trygon: nastiest MC. Can deep strike, is fleet. Tons of attacks, re-rolling hits. Really nasty in close combat.
Mawloc: MC that can pop up and hit your dudes from deep strike. Not as nasty in close combat.
So that is some of the Tyranid units. Hope it helps players that have never seen them before. I'm sure I missed some strategies etc, but I aimed for a high level overview. Any comments?