The Avatar seems a bit out of place here. Usually his best use is in a footdar list where he leads the spearhead of troops granting the 12" fearless bubble and benefiting from Fortune spam. With the new
FAQ requiring farseers to be outside vehicles to cast you can't mech it up and make him survivable.
*IF* you wanted to go full mech; a pathfinder unit to hold the home OBJ and for the Quad would be better in the long run than the 10 avengers. At least they will have range. You can drop a guide/fortune seer (basic seer) back here to use to quad and to guide the war walkers holding your line.
Eldar are specialised. Very specialised. You kinda have to pick one of the strategies and go for it. Basically you've got a few options - full mech (which has its drawbacks at the moment), full foot, wraithwall (10 wraithguard as a seer bunker and nearly unkillable troops choice, add wraithseer for full effect), or jetbike spam.
Most games will be 3-5 OBJ, so only 2 scoring units can really hurt. 3 man
GJB squads can sit in reserve till the last minute and turbo boost onto an OBJ (gaining a 4+ cover save in the process). Having the 10
DA in a serpent to nab an OBJ is ok; but Avengers are average at best. I personally get more mileage out of my 12-14 man guardian squads than
DA's; especially for return on investment.
One of the biggest things about Eldar what ever road you take is the emphasis on force multiplication. Guide and Doom are force multipliers. Beef up your units to take advantage of this:
War walkers (when running them) should be 3 strong. Gives you 2 extra hull points before the unit goes down AND gives you potentially 8 more shots. A farseers guide is better points spent guiding 24 S6 shots than guiding 16 S6 shots.
Blade storm is cool, but you won't be able to overwatch or shoot the next turn. Kinda hurts there.
I find prisms to be a bit meh. Some people swear by them; but I find for 10 points more that a Warp Hunter has a prism beat in everything except range.
Have a look around on the army list section of dakka; theres a few previous eldar lists for each of the above mentioned play styles - Mech, Wraith, Foot, Bike. Mech is ultimately the weakest of the 4. Personally, I run a fluffy (but proven to be quite scary) Wraith list (check out my battle reports for how it's been going recently). T3 on 90% of your units kicks you in the teeth; so bear that in mind.
If you want to run mech; that's about all the advice I can give as Mech is not my preference or strong suit. I wish you well though