Now most of the tactics for bringing down the FMCs will be relegated to the Nids, but I feel that the tactics can be applied to the Demons as well. Actual flyers though are still a different beast that can have similar tactics applied to but still need a bit of finesse.
Now looking at the 3 games against Nids that I played in 24 hours, I can tell you that the FMCs are a very tough group of beasts to take down. Most of the time you are going to have missiles or heavy strength shots that are going to try to ground them so the rest of your army can attack them. Unfortunately the grounding check happens at the end of the phase, so we have to be clever about bringing them to the ground.
The most obvious option is to take them out of the sky with mass fire power. This is a valid tactic since most of the FMCs are going to be Armour 3+ we can have some weapons to bring it down. Most armies are going to use either another flyer or a weapons platform with Interceptor/Skyfire. With Jink keeping these guys in the air longer, you would have to devote an entire round of shooting all weapons at the FMCs to try to bring one down. So 50% of your shots will more than likely be ignored because of Jink, and if it is low AP, then you are looking at 66.7% of your wounds being ignored. With massed firepower you should be able to take them out, especially if you have high AP and Strength.
The second choice is the ground and pound. Now this one relies on a grounding check that can come from either their turn, or yours, depending on your list. The first option is for you to Intercept them. Using an interceptor weapon once it comes on and it is within range, you could potentially drop them, but you have to get the shot through (without them jinking), then wound them (needing high strength, low AP, or massed shots), and then hope he fails at the end of his movement on a 1 or 2.
If it is a Psyker, you could potentially see it hit the ground on a failed Psychic Check. If it suffers a wound in the Psychic phase, it could ground itself before your turn. Now in your turn, you could bring it down in three potential phases. Movement, you would need to have a vector strike unit that causes a wound and could cause it to ground, in the Psychic phase, if you can do a wound by any sort of power, you can drop it to the ground, but we are more than likely going to try to kill it in the Shooting phase. Once again, there is a very good chance that it is going to survive and make its way across the board.
Yet, the strongest thing we can do is cause them to Jink. We can cause them to Jink when they come on, via Interceptor, or in the Psychic or Shooting phase. By getting them to Jink, which they have to declare before To Hit rolls are made, they get a great cover save (4+) but they can only shoot Snap Shots until the beginning of their next movement phase. This means all of the blast weapons and templates can't be fired at you, and they can only make snap shots. Not only that, but if they Jink in their phase,they are snap firing in their Psychic phase and their shooting phase that round.
So are the Flying MCs dangerous? Hell Yes they are, but we have a variety of ways to neutralize them or reduce their ability to harm us. So what do you think, can these tactics be useful? Am I too far off the deep end? What are your thoughts?