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This might fit in army lists, but I talk a good bit about why certain choices were made, so I think general Tyranid discussion is a better choice.
I do not much enjoy having lots of units of exactly the same thing. Nor do I always like my units "what is most efficient" for power playing. So, this list would provide some fodder for those who want to trim things (mostly the warrior biomorphs and the Carnifex configs), but I do enjoy it and it fits my playing style.
Hive Fleet Kohr-Ah
Vermithrax (Winged Tyrant)
Biomorphs: Acid Maw, Adrenal Glands (I), Adrenal Glands (WS), Flesh Hooks, Implant Attack, Sym Rippers, Toxic Miasma, Toxin Sacs, Wings.
Weapons: Scything Talons, Lash Whip and Bonesword
Psychic: Warp Blast
Grendel (Walking Tyrant)
Biomorphs: Acid Maw, Adrenal Glands (I), Adrenal Glands (WS), Enhanced Senses, Flesh Hooks, Implant Attack, Sym Rippers, Toxic Miasma, Toxin Sacs.
Weapons: Venom Cannon, Twin-Linked Devourers
Tyrant Guard: 3 of them, two with Lash Whips instead of Scything Talons
Quickdeaths (Leaping Warriors) - 2 broods of 4 and 1 brood of 3
Biomorphs: Adrenal Glands (I), Adrenal Glands (WS), Flesh Hooks, Ext Carapace, Leaping, Sym Rippers, Toxin Sacs
Weapons: Scything Talons and Rending Claws
Genestealers - 2 broods of 6 each
Biomorphs: Ext Carapace and Flesh Hooks
Weapons: Rending Claws and a Foul Temperment
Spinegaunts - 2 broods of 14 each
Ymir (Fire Support Carnifex)
Biomorphs: Enhanced Senses, Reinforced Chitin
Weapons: Venom Cannon and Barbed Strangler
Gamera (Infantry Killing Carnifex)
Biomorphs: Adrenal Glands (I), Adrenal Glands (WS), Flesh Hooks, Enhanced Senses, Sym Rippers, Reinforced Chitin, Spine Banks, Tail Weapon - Mace, Toxic Miasma
Weapons: Barbed Strangler and Twin-Linked Devourers
Garthim (Tank Hunting, Space Marine Shredding Horror of a Carnifex)
Biomorphs: Adrenal Glands (I), Adrenal Glands (WS), Bioplasma, Flesh Hooks, Enhanced Senses, Sym Rippers, Reinforced Chitin, Tail Weapon - Mace, Toxic Miasma, Tusked
Weapons: Venom Cannon and Crushing Claws
I have been playing mostly Chaos Space Marines lately, specifically Nurgle. However, this list is intended to be used against pretty much any opponent.
The flying Tyrant will often pick a fight against tough units, like the enemy commander or Terminators. Does a real good job of killing characters and can even hack through a Terminator squad. The lash whip helps reduce the number of attacks coming his way.
The walking Tyrant and his guard are there to soak up fire, rain some accurate fire down on vehicles and troops, and still threaten any nasty units or characters that are hanging around as the game goes into the later turns.
I just love leaping Warriors. They are possibly one of the best units we have for charging into CC. Against any power armor army, it is hard for me to judge if, point for point, which is better: Genestealers or these guys. The Genestealers will get more attacks (especially if you strip down the Warriors so they cost less), but the long charge, extended danger zone, and especially the synapse are just wonderful.
The Genestealers are there to be a threat to anything.
The Spinegaunts, well, they are there to die in droves. Just cheap wounds, when you get down to it. I use them to engage units in CC to prevent shooting and block lines of sight.
Ymir is the long distance Swiss Army knife. He goes after tanks 1st.
Gamera is a true horror for infantry, because I make sure to use him solely to go after them. The combination of ranged weapons gives him a good chance of killing several Space Marines per turn. And, if he gets too close, most anyone will think twice about going into melee with him. Especially since the WS upgrade and Toxic Miasma means he will hit Marines on a 3, while they need a 4 to strike him.
Garthim is going to be fun (just finished the conversion). He stomps forward, unloading on the enemy vehicles, then thunders into whatever enemy squad gets in his way. Despite the better choice of Scything Talons for close combat, I just had to build a model with the Crushing Claws. Love the way they look. I honestly expect that, most the time, Garthim will end up full of holes from heavy weapons before he reaches melee.
The list works best in non-escalation games. What I do is present a rolling tide of clawed horrors that come at the enemy. The goal is to keep everyone under the moving curtain of synapse (which never seems to be a problem) and present the enemy with too many target priority choices to deal with once I enter the dangerous under 18" region. The quicker units usually make a quick sprint to get into CC, to cut off shooting 1 turn earlier than if they whole mass kept together. With the winged Tyrant, Leaping Warriors, Genestealers, and Spinegaunts, it makes for a pretty rough hit. I have seen 1 of my Warrior broods shred 8 Space Marines in a single CC round.
I will not speak of the 1 escalation game when Grendal and his guard did not arrive until the final turn, which was 7. Yep, failed every single reserve roll, even rolled 1's for turns 4 through 6. Annoying. The good thing was that he came in and immediately killed a Predator that had intruded into my deployment zone and was working my flank (we were playing an Omega Recon game).
Sometimes the list does change a bit, with the addition of winged Warrior Broods (much the same as the leapers, just winged instead). Also, one of the elite slots can change to the renowed Dual-Twin-Linked Devourer Carnifex.