It's probably not enough information to talk about Eldar with just one battle against them, but I think some thoughts have occured which I think should be mentioned.

The serpents are really hard to kill, and can put out a decent amount of fire power. I would say it's easily the bst trasport in the game. With 2 necron transports as contenders. However the serpent has greater range and fire power over the barge and even railguns have a hard time dealing with a serpent due to greater cover and pen downgrades.

The problem I saw is this. You can't win with serpents alone. Well you can, but against a competetive build you will have issues. If you fire every thing you are very vulnerable, if you are too close you can be charge and assaulted to death. If you are too far you have to drop your shielding which makes you now vulnerable.

The best range is scatter/ s cannon range, and staying with that. However 3 s cannon shots is not enough to deal with +2 armour. One serpent is almost enough to kill 1 terminator. Also doing the math the most you can possibly run is 9.

The problem I saw with my game was I had enough fire power to down two a turn, while the return fire wasn't really all that devestating. Lack of ap is real hurtful.

Which means you need actual durable units to carry the day. If instead of fire dragons and dire avengers the eldar player had wraith guard I can see the game going the other way.

5 wraith guard is around 160 points and with a serpent it's hitting you for 300 points a pop. If you spam you can get 5 units for 1500 only leaving 350 for hq and heavy support. I will than say proably 2-3 units would be something. I'm a fan for a unit of d-sythes as well. You can easily drop off you army in front of the other player in turn 1. The problem is you will not be able to assault. Which means you need durable assault units.

Fortantely for you you can take a phonix lord and take him with the wraith guard. That will deter assaults into the unit and also give you a melee option.

You can use fire dragons, but being toughness 3 makes them rather easy to kill compared to a toughness of 6.

I think bikes wouldn't be a bad ideal since they can soft rend and not an easy kill, especially if a warlock gives them conceal.

I think cover denial is what is going to win games for eldar. If I had an aegis defense line it would had been even worst for the eldar player. Cover save denial and high ap weapons are needed to get the job down.

Also the deathspinner is rather good. I would reconmend at least two. Also I am not a fan of a wraithknight, but it comes off as an ideal overall heavy support. Not a fan of the Sun Knight configuration, but I can see it's use.