[Alzer]

Two vehicles are going to be pulling some pretty heavy hits from your opponent's force. I strongly suggest taking a couple more. Also your scorpions and harlequin are likely going to get shot to bits before they get anywhere. Forcing them to march across the board to engage in combat won't get you to far. Eldar rely on mobility and precision, footslogging is not kind to them.

For the scorpions, adding shadowstrike could save you the points of a waveserpent. Infiltrating/outflanking and hitting something soft your opponent is trying to hide. Also a Scorpions claw on the exarch gives them anti-armor hitting power that the chainsabres lack.

You're dumping a load of points into the Dark reaper squad, and honestly I'm not sure how much they are going to deliver in the long run. The exarch is definitely increasing the danger of the unit for your opponents, as he can ignore cover, but having only 3 of your 11 shots being able to pen through in such an expensive unit seems rough. Consider swapping crack shot for an EML, this way the exarch could hit vehicles for you. Or just drop the exarch altogether and use the unit to control firing lanes, force enemy units into cover where your harlequins and scorpions can hunt them down.

A fire prism would be a great addition to this list (it pretty much always is), extra vehicle out to support your serpents, as well as great fire support.

Most of all, your list has a devastating lack of anti-tank ability. S6 is realistically the hardest you can hit, making many vehicles more or less out of your ability to kill. War walkers could help here, however brightlances are extremely expensive on them. For anti Tank, look into a wraithlord w/ brightlance and EML. If you have the resources, all the pointy ears seem to be really big on this "fire dragons in waveserpent" unit also.

 

[Alzer]

Take another run through of The Rules if you're unfamiliar, it's good to know them.

I'm not necessarily a personal fan of Dire Avengers, since their hats bug me, but they seem to be what all the cool kids use for their Eldar armies.

I'm pretty sure the warp spyders are tougher than a vyper, and fast enough that it won't be an issue. Vypers are a bit on the flimsy side, using those points for actual grav tanks will likely get you farther than open-topped gun platforms.

Dark Angels aren't an optimal unit, but they can help you keep control of the board. People won't be sending their heavy infantry barreling through the field at you if they realise you can rip half a unit of space marines off the board in one volley. Just don't spend so much on one unit that is so focused in purpose, and rather immobile.

I don't really know about harlequins these days. Rending got "nerfed" (see also; not broken anymore). However even with invuln saves they are very prone to being rapid fired down, definitely a scalpel unit over anything else. As i've never seen them in action, it's hard for me to really say how they fare.