My Eldar buddy is really good, and thankfully has different armies (marines/nids) and thankfully I'm piecing together a Dark Eldar army (maybe I'll do ynarri to feel good about myself till codex drops)
So a couple of more things I failed to mention:
My attempts have been with these units:
Breachers in devilfish
aun'va/cadre fireblade/darkstrider with ~30-60 fire warriors
pathfinders with drone support boat (recon/pulse accl/ grav in hit / 2 shield drones to help fire warriors)
3-6 commanders, 2 or 3 taking velocity trackers (these are HUGE against Eldar, everything flies)
Vespid Stingwings (highly underrated right now, t4 s5 -2 ap is GREAT for 15pt)
crisis team with flamers
stealth teams with homing becon
I have no tried the big suits/our flies/broadsides/piranhas/tetras/ etc because I feel like putting points into big models is a risk with Eldar, because mortal wounds ignore all your defense mechanisms, and on big high point models that sucks
Against my opponent, the last couple of games I keep winning the initial die role off. This sucks, because it means that he guys to clump at least 3 objectives because he picks deployment (my 2 cents, I think player that goes second should choose side, like warmachine. I feel like a game can be decided turn 1 with choose deployment/zones + going first, but thats me...)
Game plays like this: Eldar player picks side/places objectives/ goes first - easily can happen with winning the first die roll and losing the to go first and seizing initiative. Worst case scenario honestly, because it probably means you need to take ground, which I have found is very, very difficult for these reasons:
1) Rangers infiltrate at the start of the first battle round. They go everywhere thats not your deployment zone. Relic mission is a joke, unless you have a stealth team. They are also durable as all hell, did I mention that? They have a stratagem that makes them only hit on 6, which ruins us, as any high ap shooting you want to put into them doesn't go through, or maybe kills 1 dude. 3+ in cover is great.
2) Eldar have a stratagem that allows them to redeploy 3 units. You cannot out position them in deployment with clever uses of drops if they know what they are doing.
3) Going with tempo - wave serpents go where they want, and they WILL drop off there cargo where they want. That cargo, if it wants too, can move twice because of a psychic buff. That wave serpent is going to then discharge its shield and throw more mortal wounds
4) Our defense in the psychic phase is Sense of stone 6+ FNP , savoir protocol, and bodyguards. Firewarriors die in droves to pyschic smites, no to mention things like doom, fortune, guide, conceal/reveal. All of these things fix dice really well for Eldar, re-rolling wounds, a further -1 to hit, +1 to there saves (rangers now at 2+ in cover), re-rolling to hit, etc.
5) Our dice fixing is trashed. -1 to hit makes pathfinders hit on 5 without moving. Rangers aren't a good target, so why not shoot the wave serprent? Nope, also 6 to hit, because why would you ever not advance a transport with a payload inside? and can become unhittable with another 1 cp stratagem, "lighting reflex" making it another -1 to hit (for -3). You can't really move with pathfinders, so you need markerlight drones (haven't put them to the table yet, just theory crafting)
6) Without our dice fixers, and our native BS4, shooting at range is very, very, very inefficient. Think 60 pulse rifle shots into 6 rangers killing 3 or 2.
7) Getting rid of the psychers is very very hard against a player that can position well, doubly so when the new chapter approved comes out with the targeting rule update. Sniper drones are....very expensive points wise to become reliable (drone controller, + firesight marksmen + drones themselves) and are a glorified pulse rifle shot which CAN wound, but has to get through, and doing that en-mass will take up a lot of points from your army.
8) The high mobility of eldar allows them to send waves of units at you. Often the chaff draws your army into an engagement that it cannot ignore because all eldar units deserve attention due to the elite nature of the army.The crux is that since the eldar player is piling into you/assaulting you first, they dictate tempo so well from that point on. Getting in close to the targets you want becomes difficult, as anything that risks to go in close that can kill these are commanders. Commanders, which, can die to 1 good smite. (warp charge 10. Eldar can buff that with stratagems + equipment, so +2, which means 8 on 2d6 does a big smite)
You get mired down while the eldar back field (reapers, fire prism, rangers, psychers, etc.) are free to cap objectives/hold ground. Leaving you to, deal with chaff and not get the ground that YOU need to win the game.
Its hard to take them down on the approach due to the modifiers (which, in a game of d6 I think is a terrible mechanic,I digress...), its hard to gain footing once they get stuck in, and its hard to send out elite units to take care of things because if they do not 1 turn whatever they need to do, they will get smited down.
Getting in close is great, but please take a Darkstrider if you are working with infantry. That fighting retreat is very, very good. It can win you games by getting your troops back into the fight.
I have not tried the sunshark bomber against Eldar, but I don't think it would fair to well because of these reasons:
Dark reapers ALWAYS hit on 3+, and do big damage. Being an airplane, it most likely will fly into the enemy back line...and then become the closest unit to the psycher, and have horrible things done to it. 12w won't last long against a barrage of smites. It would solve many problems however, but it becomes smite bait. Its a damn shame we have NOTHING to help counter psychers besides passive FNP etc.
I don't mean to sound gloomy or anything, but its a tough battle going index vs codex.
However, I'll leave on a positive note. If you can see an eldar psycher out of position (the spirit seer is the key, not the 2w warlock) kill it. Kill it with fusion blasters if you have to. Its such a force multiplier for the Eldar army, and such a ruiner for yours, that killing it early in the game might be worth a commander, or that might be crazy talk.
I'll dump more knowledge as it comes to me, hope this helps y'all