|Don't let its looks fool you, THAT BABY IS MEAN!!!|
The Good News: I think most marine armies are going to have trouble with them. Why? Because Gk troops shoot like hell and still have power weapons to boot. Generally GK troops can put other power armored troops to shame. Ya they are more expensive, but having troops that can actually do something is a plus. (Unless you’re grots, then it's ok). Of course to us xenos folk, they are still marines, and that is where we beat them.
Tons and Tons of Anti-Infantry firepower, I mean seriously LOTS!!!
Generally you’re looking at the entire army having S5, power weapon attacks that often hit at I6. Gross.
HQ can carry rad grenades that reduce your toughness and even cause your own models to attack themselves.
Entire army has force weapons
Every unit has great psychic powers that can potentially win them the game
Warp Quake: I don't call broken, often. Never in fact. But this is.
Things that absolutely rape daemons and psykers
Expensive, very very expensive
Relatively low weapon skills, with 4 on most and 5 only on the uber expensive
Low number of attacks, most only have one with the best only having two
Difficulty with heavy armor. Most Gk armies will struggle with AV 13 or 14
Most guns in army 24" range
Look for Holes: Building Balanced GK army is tough. It is very easy to take too many shiny toys and that leaves GK armies underperforming in one area or the other. Find the weakness and take advantage of it.
Psychic Limitations: Most Grey knight squads can only cast one psychic phase per turn and activating force weapons counts as using a psychic power. This means while GK have many tools at their disposal, they can only use one at a time. During an assault, most if not all power must be cast before ANY blows are struck, including yours. Plan for this during game play to truly gage a squad’s capability
Assault Them, Especially in numbers: Gk may look scary in assault but due to their WS4 and low number of attacks they are often underwhelming in assault. However they are much scarier at range. Assaulting them keeps you safe from taking 20 X S5, or 4 X S6 flamers to the face. That’s a good thing.
Find the HQ and the RAD grenades: Having an HQ attach to a squad greatly bolsters a squads ability to do damage in assault, allot due to fact that it allows the squad to effectively cast two powers a phase, since the squad can 'piggy-back' on an HQ hammer hand. Couple this with Rad Grenades and you can very well fine which squad your opponent is planning on getting into combat.
Over-extension and counter attack: They vehicles that GK can move and assault out of are Stormravens and Land Raiders. What does this mean? Well they want a nasty assault unit they need a nasty squad, with a nasty HQ, with a nasty transport, that usually all totals around 600 to 700 pts. This usually means there is never more than one and the most assault support said squad is going to get is a Dreadknight. With that many points in a unit, it can't just sit back. It will very often over extend itself. Many players are turning to the Modarak squad for in-your-face-ness, but I still feel this is often and overextension. Be pre-pared and be able to bring down the weight of your army if it over extends.
Death stars: Take a Deathstar, something mean, something quick. Now, don't try to take out his death star, Cuz you'll probably lose that fight. Try to put it into the rest of his army. This will take out his fire base which is very important if you are troop based like nids or orks.
Plan for the Dreadknight: I know it's gotten some crap because of the model, but competitive players who are looking to add speed, firepower, and assault capabilities in a durable package simply won't be able to pass it up. Have something in your list that can handle it.
Plan for the Shunt: Dreadknights and Interceptors have the ability to move 30" once per game. Plan ahead and don't forget about it.
Don't Deepstrike Anything!!!: Seriously. Nothing. Warp quake has absoutlely ruined the concept of deepstriking. If you can't, not deepstrike it, take it outta the list.
Well there you have it. What are some ways not discussed here that you are finding to take these shiny marines down? Tune in next time as I attempt the monumental task of writing a very hard article: Anti-GK tactica: Tyranids!!!