I did write a long reply to this last night and the Interweb ate it.
Second attempt...
Farseer, stones, warding, doom, fortune, guide 165
Drop one of the powers. Unless you go for a Seer Council (which I would strongly suggest you don't) then I would drop Fortune from this list.
Just sit at the back guiding the war walkers and dooming their target.
10 dire avengers, exarch, defend, bladestorm, tw, catapults 167
wave serpent, stones, tw/l suri cannons 110
This unit is absolutely fine, though arguably you shouldn't need defend if you use them carefully.
I would also suggest finding the points to put EML and chin shuricannons on you Serpents.
This setup is golden against horde armies, since the plasma missile is defensive, allowing you to fire both heavy weapons on the move. Also plasma is pinning, which both guard and nids are susceptible too if you can isolate swarms from their HQ / Synapse.
3 scatter walkers 180
Yup. Guide them. Shoot stuff.
6 fire dragons 96
falcon, pl, scatter, holofields 175
Fine. For versatility theis squad could take an Exarch with a flamer, but it's not essential.
I would also get SStones on the Falcon. In fact you
must put SStones on that Falcon.
10 howling banshees, exarch, executioner 182
wave serpent, tw/l suri cannons, stones 110
My suggestion here is to drop the idea of an elite close combat unit entirely.
Use tripple flamer storm guardians instead. They're almost as good against guard and the flamers will make such a dent in a nid swarm that you'll hardly have to get into combat.
They're also a scoring unit, which you're a bit light on at the moment.
10 guardians, scatter laser, warlock, embolden 125
6 rangers 114
Fine. Keep them in cover as the core of your firebase. Again, the ranger's pinning weapons should be targeting either Monstrous Creatures or isolated mooks that will likely fail pinning tests.
wraithlord, lance, eml, flamers 155
And keep this guy with them to deter assaulters.
8 swooping hawks, exarch, hawks talon, skyleap 205
8 warp spiders, exarch, withdraw, powerblades 213
I like both of the these units.
I'd recommend the Sun Rifle on the Hawk Exarch to push out lots of pinning tests. They're not reliable, but if you force enough of them then you opponent
will fail some.
Against Bugs you hold back and shoot, then move in with your short range units to mop up anything that gets close.
Against Guard you need to be more proactive since you won't win a long ranged fire fight against them. Advance the assault elements under covering fire and once you're in amongst the troops you should outmatch him.