Well, I'm not an Eldar player, but I spent years playing against Eldar and I can tell you what a worst-case scenario would be from an opponents point of view.
The Eldar are a unique army in that every piece of it is highly specialized, and so the key to using the Eldar isnt to use every unit for exactly what it was meant to do. Many Eldar players try to cheese out taking 3 wraithlords and lots of Dark Reapers and star cannons everywhere. That can be effective but only against one type of army and only if they havent already figured you out. From your posts its pretty obvious that youre not that type of player anyway.
Ok, a mechanized Eldar army is good, but the only realy way to get it done is Saim-Han. Wave Serpents can be a great idea when you have one or two, but trying to put your whole army in Wave Serpents is right out of the question. Theyre very expensive, dont have much in terms of weaponry, and dont contribue to your overall strategy except to get that squad across the field in one turn. For any other army, a Wave Serpent would be amazing, but when everything you have can go twice as fast as everything the enemy had and for the most part can ignore terrain, Wave Serpents arent all that great. Good for one or two units, but in massive quantities theyre a point sink.
Here is my advice to you and all Eldar players who are constructing army lists out there for what your opponent really doesnt want to see on the other side of the board from themselves.
First, avoid the urge to build your army from the Aspect Warriors up! Youre not Space Marines, youre more like Imperial Guard with Space Marine allies (which is one of the deadliest team ups in the game) and so dont fight it. Start simple. Farseer HQ and two large squads of guardians are essential. So many Eldar armies forget these core units and go straight for Aspect Warriors. Go ahead, add warlocks to the Guardians, and while youre at it add a weapon platform to the guardian defenders too. Now feel free to add your Rangers, but take a full squad too because youre going to need it. Now you have a solid core. These 40 guardians are going to be your multi-purpose units, able to handle any combat role. This is why theyre so important. They can take a lot of hits and they can do anything. The Aspects that you do take are going to need them.
Ok, so the idea of your army is mechanized, so you want to get across the board quick. Here's where the wave serpents come in handy, but you dont want too many. Theyre large and get in the way and in large numbers they become problematic. So I say take 2 wave serpents. There are only two units in the whole Eldar list that I would say actually require a wave serpent transport. Striking Scorpions dont have fleet of foot and will take massive casualties hoofing it, but once theyre in assault theyre nasty! So dont wait, get them stuck in first using a Wave Serpent transport, but be sure to take the Exarch. The other unit is Fire Dragons. Fire Dragons are really overlooked by a lot of Eldar players, but 10 of them firing into any unit is going to cause DAMAGE! Remember that they can fire on the turn they get out of the serpent too. However, selecting both of these to put in your two wave serpents will be rough on you. Pick two units either Fire Dragons or Striking Scorpions and use them at the same time.
Now to finish out your mobile forces. Guardian jetbikes are great, and used in large quantities they can be very damaging and put incredible pressure on your foes. I'd strongly advise two squads of jetbikes. One full squadron of Vypers is a must also, as they will add highly mobile fire support.
Heavy support can be the most imporant selections in an Eldar force. A Falcon is a must! A wraithlord is a normal choice as well, but with all the plasma and hidden powerfists abound its not going to be too effective. If youre going for raw firepower, a second Falcon or one or two squadrons of war walkers can be positively withering!
So far you have an HQ, a solid core of troops, the ability to get 4 hard units across the table and in your enemy's face in the first or second turn, and a withering fire base. What more could an Eldar player ask for? If you still have points burning a hole in your pocket, buy more guardians! You can never have too many guardians. If you want, a unit of Howling Banshees is good too, but run them along the ground behind the Wave Serpents and use the front units to lock in the Bashees' target in assault for your charge.
This is a well balanced army, able to control the field and deal with any army. A good idea is to swoop forward with your Serpents, Bikes, and Vypers and get in their faces. Move forward with the guardians and the weapons platforms along with the Farseer and follow up, while the Falcon and walkers unload all they can ignoring enemy fire. Think of it this way, the fast forces are the nail and the guardians are the hammer. You create a weakness in the line with the Aspect Warriors and then exploit it with the guardians and your farseer while anything not getting clobbered gets shot. This is very difficult to defend againt and even more difficult to attack.