I've had quite a bit of successive versus nids and my dire avengers were key in all my victories. Unless you're dealing with a flying circus (a ton of flying hive tyrants). The beauty with avengers is their massive amount of shuriken shots that can cause rending. This is fantastic against a nid player with lots of high toughness, monstrous creatures. The ton of shuriken fire is not too shabby against a horde army either. Taking a vanilla avenger shrine, 30 avengers will dish out 60 shots and once per game you can boost that to 90. I've taken out a tervigon per turn with this formation. It's versatile, quick with the use of battle focus and most opponents won't place priority in removing them.
You actually have a good model base to run a footdar army i.e. all infantry, no vehicles. This is my favourite type of eldar to play. With battle focus, they are so quick that you don't need to waste points on transports.
750 point games are going to be very tough for eldar. Once you get up to the 1500 point level, you'll notice a remarkable improvement in how your army performs. Many units in the eldar barracks are specialist units and work in conjunction with other units. You don't have many jack-of-all-trades units with eldar.
Technically both lists you have posted are unbound, which by no means is illegal and many players play these lists at really low point games or really high point games. Just make sure your opponent is okay with it before you play. To have a CAD you need 1 HQ and 2 Troop. This is where I think part of your issue is with getting wrecked in a lot of games. In an unbound army, especially at low points, peopel tend to put in really expensive units that you rely heavily on, while your opponent is able to field a much larger army. People also end up making very unbalanced armies. An army doesn't need to be completely balanced, but if it focuses too much in one aspect without mitigating its weaknesses then the average army will most likely beat it. The lists you have posted are of very small, short range units that are easily split up and isolate. They don't play very well to the eldar strength of speed.
What I would start with is just a normal CAD at 750 points and build from there. Sure formations give you some great bonuses, but you may be short changing your army to try and fit one in at 750 points.
I'd try something like this:
Spiritseer (70)
Dire Avengers x 8 (104)
Dire Avengers x 8 (104)
Rangers x 5 (60)
Rangers x 5 (60)
Scorpions x9 + exarch w/ claw (210)
Wraithlord x2 flamers, Brightlance (140)
[748]
Now that's just working on what you have. I'd setup the spiritseer in a unit of avengers with the wraithlord beside them. The units of rangers could stay in reserve and outflank or setup somewhere on the opposite sides of the board. The other dire avengers would setup in cover, center board. The scorpions can setup center board as well or outflank. Your cover saves with all that stealth should keep your guys alive for a while while you press your attack. Your opponent will most likely focus on the wraithlord, being the big, bad meany on your side. Use the primaris power to keep yourself safe unless you roll well and an opportunity shows up that is too good to miss.
Hope that helps, I'd look into investing into some more avengers and some warp spiders. I'd also keep an eye out for an avatar once you get high enough points, as they work very well in a foot army.
Good luck, I hope the advice helps.
*edit* you can put the avengers and scorpions in a formation with this list because you have the two units of rangers as troops