Let me start this off with a small preface: those of you who've read my articles in the past, in particular those on Tyranids, know that I don't generally subscribe to the "sky is falling" nonsense that pervades so much of the 40K blogging and forum community. Tyranids are not horrible and worthless; their MCs, while sometimes overcosted, are mostly just fine and usable and there are only a scant few worthless units in the codex. I've done a fairly extensive exploration of alternate builds, obscure units, and various nonstandard Tyranid ideas of all sorts, and I've sent my Tyranids to more than just a few tournaments with a fair degree of success- and the failures have been my fault more than the codex's every time. So I hope you'll understand that when I write this article, it's not simply jumping aboard the "omg GK so gud!" bandwagon that is making its rounds through the community these days, it's because I have sat down and taken what I feel to be a fair assessment of the situation. With that said...
The Grey Knights codex marks the beginning of what I fear to be a waning phase for the Tyranid codex, which is all the more unfortunate because it's such a recent release that we can't expect to see another for at least two to three years. It's not the very start- from the time it was released, the book has had some issues, not the least of which being overcrowding in the Elite slot, but these were compensatable and left several major builds open even in high-level play. The release of the FAQ, however, effectively shattered any kind of reserves strategy and made the Doom effectively impotent, along with our psychic defense. However, the "primary" build for the codex was largely untouched and continued to be a pretty viable contender in tournaments even against heavy mech armies. Although slotted into a fairly limited range of top-end builds, Tyranids are/were still a strong army in most ways.
However, with the release of the Grey Knights book, the pendulum is starting to gain momentum. Tyranids have no easy answers to many of Grey Knight's primary strategies and GK have innate resistance to many of Tyranid's strategies. There is an inherent inequality between the two armies on the table, and one that I fear will spell the end of Tyranids as a competitive choice at tournaments.
That comes off as rather over-dramatic, I'm sure, but to clarify: against non-GK armies, nothing has changed. Tyranids are still able to hold their own against IG, SW, BA, DE, SM, and all the other books, assuming that you are playing on a good table, don't have silly missions/tournament rules, etc. However, Tyranids will be fighting a steep uphill battle against anything resembling a well-designed Grey Knights army, as they suffer from too many inherent weaknesses against them.
A good, balanced army should be able to stand a chance against every other top army in the field; if you struggle fantastically to beat one of the other main contenders, that is a sign that you need to tweak your army design. ("Rock" armies and such that just intend to present a ridiculous threat and hope to ride to victory on its back are different, hence why I do not consider them "good" armies, as they will too often face up against their nemesis and fold. This doesn't stop them from getting into the top ranks, though.) With the addition of GK, Tyranids can no longer be expected to fit this criterion: any build which functions effectively against GK will be largely worthless against the rest of the field, and vice versa.
The How and Why
I've mentioned that Grey Knights are more or less perfectly poised to tear Tyranids up, and so as to support my position I'm going to elucidate on why I think this is true. It's nothing as simple as a single piece of wargear or rule option, but rather a combination of multiple different factors that work together to effectively demolish the Tyranid strategy. Where other armies may/will have options to fight these factors, Tyranids, with their book lacking in many areas, cannot, and just as importantly many of them seem to attack Tyranids' key weaknesses in ways that aren't really a problem for other armies.
1. Instant-kills
One of Grey Knights' big strengths is that they have extensive access to ways to eliminate "elite" armies- Force Weapons all around make them able to cut down anything that relies on an armor save for survivability and even multiwound models aren't safe from them, as they can trigger their NFW's to kill it. This is annoying to most MEQ armies, as they can eliminate important HQ models with their basic troopers and GK basic units will almost always trump their counterparts in a fight, but for Tyranids it's disastrous. Lacking any invulnerable saves (unlike other codices) the ability to chop through armor saves is dangerous enough, but our models are almost all reliant on having big piles of wounds in their place to have a measure of survivability, and the various insta-kill mechanics highlight the lack of Eternal Warrior in our book anywhere.
It's not simply the NFW's even; Inquisitors can get Null Rods. Librarians can get Warp Rift. Brotherhood Champs get Heroic Sacrifice. Valeria has the Hyperstone Maze. The Callidius has her phase sword. More than any other codex, Grey Knights can slaughter multiwound models with relative ease, and many/most of these also mitigate or bypass the high toughness that we rely on to protect us from a lot of things, all the more so because of Rad Grenades, Hammerhand, Might, or Daemonhammers.
2. Extensive anti-infantry
This is hardly the first army we've seen with ways to cut down our monstrous creatures and other multiwound models, though. Dark Eldar and Space Wolves both have good ways of doing this and we can mitigate them reasonably effectively while maintaining the strong offensive front we need in order to be effective because those same armies had trouble dealing with one of our other main strengths, namely the ability to flood the battlefield with a tide of resilient, dangerous small critters. Neither army particularly appreciated being drowned in Termagants, especially when they were backed by Catalyst, Paroxysm, or both, and this ended up being our strategy against a variety of threats that our MCs weren't really equipped to handle.
GK, however, come standard with a Storm Bolter, which is extremely bad because it not only shoots harder than a Bolter at most ranges, it also is 100% mobile, which negates another Tyranid advantage. In fact, even their "heavy" weapons are at least partially mobile, allowing them to effectively shoot on the move in much the same way we do. Moreover, those same weapons achieve their ends through having high-strength and high ROF (along with Rending), which allow them to function as secondary anti-infantry guns. Combined, these mean that most of their units can put out a prodigious hail of shots, allowing them to cut down our smaller units in order to expose the big bugs behind them and apply their ID weapons as explained above. Not only Storm Bolters and Psycannons but also Autocannons, Heavy Bolters, Incinerators, the Purifier aura, Acolytes with Boltguns/SBs, etc, all contribute to this weight of fire.
It is this one-two punch that GK can deliver that forms the basis of their strength against Tyranids- we cannot rely on our MCs or multiwound models to defeat them, because they have incredibly effective ways to kill those. At the same time, we cannot rely on our smaller critters to defeat them, because virtually everything in their army is also quite proficient at cutting them down.
3. Fortitude
Unlike the above points, this boils down to a single, devastating ability: Grey Knight vehicles are all but immune to being stunned or shaken. Against other armies this is certainly strong- after all, the concept of suppression fire is an important one, and being resistant/immune to it is a very big deal under any circumstances. However, Tyranids are specifically reliant on it for several reasons; for one, we have a variety of weapons that take penalties to the damage table, such as Venom Cannons and Devourers, both of which fulfill useful suppression roles but have very little chance of actually killing a tank. For two, we lack the same abundance of anti-tank guns that many armies can field, so Tyranids are often forced to suppress some targets targets use assaults to wreck them in later turns. Suppression is key to the Tyranid strategy, because it allows us to mitigate the effects of enemy transports by negate two units (the transport + whatever's inside it) by shooting once, and if they choose to disembark, they are effectively doing our job for us.
GK, however, don't have that problem. 90% of the time, they simply shrug off the effects and keep doing what they were doing anyways, and even a Perils will probably do little but continue to keep them in place without lasting effect. Combined with the short range of our psychic defense, GK can generally just sort of walk away from us and shrug off any but the heaviest of AT fire we put out, which makes the necessary step of de-meching them extremely difficult.
4. Grenades and other runners-up.
There are tons of other little factors that, while not crippling, make fighting GK extremely difficult. Rad and Psychotroke Grenades, for example, can make a fight VERY difficult for Tyranids- the former can reduce MCs to T5 (which doubles the chances of most Force Weapons killing them), while the latter makes any kind of assault potentially very dangerous- hitting yourself, striking last, etc, are all potentially crippling effects, and neither type of grenade can be mitigated by assaulting the unit, as they work both offensively AND defensively. On top of that, every GK unit carries Psyk-Out Grenades, which shut down many of our big critters, and have extensive other anti-psyker tech that can cause havoc for some of our mainline units. Sanctuary further exacerbates our own lack of grenades of any kind and punishes large Termagant units at the same time. Dreadknights make a mockery of most our MCs, getting not only 2+ saves and an Invuln but S10, heavy weapons, and better-than-Wings as well. and for a drastically lower price.
All of this, of course, is talking about the army's components in isolation, which is hardly definitive proof of anything. (It's the reason that Grey Hunters are not simply extra-better Tacticals, for example.) Much as I would love to go into the details of how GK armies function and what some of their build options are- and how these relate to Tyranids- such a discussion is probably beyond the scope of even this increasingly-lengthy article, and I expect that it would be limited by my own knowledge of exactly how GK function on the tabletop at this point. I am sufficiently confident of what I do know, however, to say that the above advantages are all functional within the context of a normal Grey Knights army, being as they are largely core to the design of the army, so virtually any build one can conceive of will still be able to take full advantage of them.
This should also not be misconstrued as "Tyranids can't beat GK"; they can, it's just an unusually hard fight for them, more on par with Necrons-vs-anything than matchups of the various 5E codices against each other.
