Your best friend, when playing against grey knights, is numbers.
Grey kinghts are an elite, high cost, low model count army. If you present them with TONS of gibblies, all their heavy firepower is wasted.
Normally I'd suggest my tournament geared army (seen here:
http://www.dakkadakka.com/dakkaforum/posts/list/585262.page) however since you know your opponent I'd suggest: forgive me for the formatting, I used battle scribe and I am on an iPad so deleting all the extra info is super tedious.
+
HQ + (480pts)
* Deathleaper (130pts) (Codex: Tyranids p88)
* Tyranid Prime (155pts) (Codex: Tyranids p41)
(Alpha Warrior, Independent Character, Shadow in the Warp, Synapse Creature, Very Bulky)
Boneswords (), Flesh Hooks (), Scything Talons, Toxin Sacs ()
* Tyranid Prime (195pts) (Codex: Tyranids p41)
(Alpha Warrior, Independent Character, Shadow in the Warp, Synapse Creature, Very Bulky)
Boneswords (), Flesh Hooks (), Scything Talons, The Norn Crown () (*), Toxin Sacs ()
+ Elites + (280pts)
* Venomthrope Brood (90pts) (Codex: Tyranids p88)
* Venomthrope Brood (90pts) (Codex: Tyranids p88)
* Zoanthrope Brood (100pts) (Codex: Tyranids p89)
2x Zoanthrope (100pts) (Brotherhood of Psychers, Psychic Brood, Shadow in the Warp, Synapse Creature, Very Bulky, Warp Field)
+ Troops + (620pts)
* Termagant Brood (160pts) (Codex: Tyranids p91)
* 15x Devourer Termagant (120pts)
(Instinctive Behaviour - Lurk, Move Through Cover)
15x Devourer ()
* 10x Fleshborer Termagant (40pts)
(Instinctive Behaviour - Lurk, Move Through Cover)
10x Fleshborer
* Termagant Brood (160pts) (Codex: Tyranids p91)
* 15x Devourer Termagant (120pts)
(Instinctive Behaviour - Lurk, Move Through Cover)
15x Devourer ()
* 10x Fleshborer Termagant (40pts)
(Instinctive Behaviour - Lurk, Move Through Cover)
10x Fleshborer
* Tyranid Warrior Brood (100pts) (Codex: Tyranids p41)
Barbed Strangler
* Tyranid Warrior Brood (100pts) (Codex: Tyranids p41)
Barbed Strangler
* Tyranid Warrior Brood (100pts) (Codex: Tyranids p41)
Barbed Strangler
FA
* Gargoyle Brood (120pts) (Codex: Tyranids p93)
* Gargoyle Brood (120pts) (Codex: Tyranids p93)
HS
* Mawloc (140pts) (Codex: Tyranids p95)
(Burrow, Deep Strike, Fearless, Hit and Run, Instinctive Behaviour - Feed, Terror From the Deep)
* Trygon Prime (240pts) (Codex: Tyranids p95)
(Deep Strike, Fleet, Shadow in the Warp, Subterranean Assault, Synapse Creature)
Bio-electric Pulse with Containment Spines, 2x Scything Talons, Toxin Sacs
Deep strike Trygon and bring Termagants with non warlord prime attached in subterranean assault. Infiltrate deathleaper to get him into position for Mawlock TFtD. Walk the warlord norm crown prime with other termagant unit, beside warrior units. Venomthrope units smattered in and Zoans to threaten
LR.
Gargoyles are cheap fast screen and because of norn crown, you'll be able to keep synapse on them for most the time, until Trygon Prime enters.
Oh and target Coteaz with the It's after me rule.