Ok, this is going to sound awfully rude of me, but I msut say it anyway. Your foot slogging Council (minus the extra Seer as its an illegal move) is going to get raped, and you certainly don't have enough troops to back you up or force your opponent into a corner. Wave Serpents with a little focused fire go down like a sack of poo and will leave your council stranded on foot. Ideally in a force like this you would want two units of
DA's so you can make damn sure your focused fire is going to really count. To give you an idea of how
id beat this here is a brief picture:
1. Move my units out of your firing lanes for the Rangers/Pathfinders.
2. Lure you around whilst putting a few focused shots into your transports, either destroying them or to some efect making them redundant
3. With your fire lanes being of no use and your Seer Council / Dire Avengers / Fire Dragons (any of) stranded, and the rest of your army trying to catch up, your primary asset and lack of troops has now put you in a place where I can pick you off piece-meal until your big boys show up.
4. By this time your forces are possibly scattered, being pulled apart by focused fire with you unable to recover ground in a hurry and playing the whole "contest everything I can" game because you have two units (Rangers etc) moving, unable to fire trying to contest/take objectives, and one (
DA's) that is being pulled apart without getting into the fight
In short I can't see a solid center of gravity that pulls your force together to fight as a whole, just looks like it could end up like a "pool table break" with everything ending up all over the board, chasing the enemy down, when they should be chasing you. The more turns your opponent rolls more dice than you, the worse off you're going to be by the turn you get close enough to start rolling dice yourself. Looks like the majority of your army (Rangers, Pathfinders, Wraithlords, Avatar, Council etc) is going to end up running the whole time and not rolling dice if the enemy uses cover properly, and you dont have enough units to fight attrition style, and not enough mobility to put everything in his face on the first turn and survive.
Arm the Wave Serpents up with
TL-
BL and focus on the armour straight up, so you dont have to worry about it later when your troops are mopping up the enemy troops, make your range work for you early on, else they get the first shot and your transport is wasted. Your army focus seems a little disjointed in the fact you have half Mech half Slogger, and the way its configured you will get picked off piece-meal unless your serpents stay close to your anvil (Avatar and Lords), which is not what they are designed for really... speed is their asset and main focus.
Personally,
id ditch the rangers and throw in another
DA unit, perhaps break the Council down a bit and stick them on bikes, then they don't rely on a "lucky shot, you're stranded" transport. Putting them on bikes gives you a T4 3+/4+
inv (re-rollable) unit with an extra 6" move in the Assault phase to increase their survivability. Throwing in a few spears will also make your Dragon's redundant as they can hunt armour faster and keep at range without the inevitable suicide squad stigma that Dragon's have become accustomed to. Although, reading that you want it on foot, heck go for it, just thought
id bring up some points though