*Rolls up sleeve*
Here I go again!
Your HQ; that's a LOT of points, and the Avatar is basically a gratuitous addition to it. I understand that it's a cool model and all, but if you want a mobile army, drop it. I'll tell you what to get in its stead soon enough.
On a practical note, exchange the Ghosthelms for Runes of Witnessing. They're far more useful for their points. Demon attacks aren't that big a deal, I'll have you know.
Also, get rid of one of those Destructors and get Embolden instead. You really don't want that squad overrun, do you?
Your Elites are expensive, but then again, Elites are the Eldar's game. Only one note; if you're getting a Scorpion's Claw, don't get a crushing blow. There are only two units that that extra point of strength will matter for (remember, it's not doubled with the Claw), and that's the Talos and Wraithlord. Simply put, either exchange the Scorpion's Claw for a Biting Blade or drop the Crushing Blow.
The Howling Banshees look pretty good.
On another practical note, I don't really see the purpose of the Striking Scorpions. The Seer Council can take care of high toughness low save models, and the Banshees the converse; low toughness high save models. But do you really want another unit simply to simulate the Scorpions? It seems pointless.
You don't have enough troops. First, max out those squads; it'll help your composition score (which, sadly, will be pitifully low so far); now give those Warlocks Singing Spears. However, I would suggest a drastic change. If you drop the Scorpions, you could invest in some Rangers, giving you some much needed fire support.
And finally, no Wraithlord? Normally I would praise you for such an action. But this time, it's silly not to take one. Consider it; all you'll have on the board at the start of the battle are three Wave Serpents and two Guardians Squads. DEFINITELY perfect for shootier armies. If you were to play against a shooty army, you'll go down very, very rapidly. And if you go second... your skimmers can be penetrated. Ouch.
Finally, when you get to the thick of it, you have to realize that you only have six squads. That won't be too useful for claiming objectives. And concerning the Seer Council offensively; it takes YEARS for it to kill a single squad. While your enemy sacrifices Grots and Cultists, they'll be claiming objectives while SEVEN HUNDRED points of your army are in one stupid delaying action. Quite silly, really. Besides, imagine the effects of one Lascannon in a Devastator squad in this army. Deadly. If your army has a brain, they'll smack your Seer Council Wave Serpent. And since your plan moves you 24" towards the enemy, they'll get a lot of free shots. And that's going to hurt.
