Similar to my post on War Walker configurations, I thought I'd write up a summary of the more obvious AA options in the new Eldar Codex. If you do these calculations on your own, and believe that you've found errors in my work, please let me know so that 1) I can correct the number in this post, 2) I can improve my methods, and 3) I can share more accurate and useful unit evaluation tools with you in the future. In particular, if you have a proposal for another Eldar unit with decent AA, I'll add it to this post as an edit.
I've listed each unit with its points spent per hull point removed, as well as the points spent per Explodes! result, against AV10, AV11, and AV12. The first thing you should notice is the average number of points to cause an explosion is so high that each unit is has almost certainly destroyed the vehicle by stripping hull points before an explosion can occur. That is not to say that vehicles don't explode, but it is rare enough that it isn't worth claiming that weapon AP is a significant factor against vehicles when you are choosing units for a list. The second point of interest is that, unlike the War Walker post, I have not provided the actual numbers for these units attacking other targets; it is sufficient to say that the War Walker configurations are inferior to Scatterlaser + Starcannon build, the Dark Reapers can kill most infantry and MCs efficiently, the Warp Spiders can shred almost anything, Wave Serpents are deadlier than Fire Prisms to almost all targets (I'm not kidding), and Crimson Hunters are best used against your opponent's toughest units. Finally, the AA calculations contain no saves of any kind from the Flyers; if you want to transform the numbers for a 5+ save, multiply the values I've given by 1.5 (because the number of glancing/penetrating hits is multiplied by 2/3). Just don't do it for the Reapers unless you're shooting a Heldrake, Nightwing Interceptor, Phoenix Bomber, anything cloaked by a Ravenwing Darkshroud (etc.).
Note: due to the large numbers of platforms that can sport multiple S6 weapons with different APs, ranges, and rates of fire, I will not add any to the list beyond the War Walkers with 2 Scatterlasers; that means no Vyper, Hornet, or other War Walker shenanigans. The transformations are really easy to do, so work them out yourself. For example, to figure it out the numbers for a Hornet with 2 Scatterlasers, divide the points cost of said Hornet by the cost the War Walker with 2 Scatterlasers, then multiply the result by the pts/HP removed from each target by the War Walker. Don't bother with the pts/Explodes! number, since it's not worth planning around. Due to weapon pricing, all other matched pairs of S6 weapons on a War Walker will be less efficient at killing Flyers than the 2 Scatterlasers in both the regular and re-rolled circumstances. Mixed S6 weapons that include a Scatterlaser will be more efficient than 2 Scatterlasers without a re-roll, but less efficient than 2 Scatterlasers with a re-roll.
War Walkers w/2 Eldar Missile Launchers, Flakk Missiles (x2)AV10 123.75/HP; 990.00/EX
AV11 165.00/HP; 1485.00/EX
AV12 247.50/HP; 2970.00/EX
It turns out that the only War Walker configuration Phil Kelly built from the ground up to kill Flyers is also the worst tool for the job. Not only is this unit an embarrassing testament to Mr. Kelly's inability to handle simple fractions, it's extortionate cost also makes it a poor choice against all other targets. Pinning does not redeem the Eldar Missile Launcher's almost insulting expense.
War Walkers w/2 ScatterlasersAV10 105.00/HP; 918.75/EX
AV11 157.50/HP; 1837.50/EX
AV12 472.50/HP; Glance Only/EX
With the exception of shooting AV12, these War Walkers are actually better than those with 2 Eldar Missile Launchers, especially considering how much better they fare against other targets. If you have a Farseer to give these War Walkers a re-roll, they become a good anti-air option (see the other entry farther down this list), but otherwise I'd recommend sticking with a Scatterlaser + Starcannon for other targets, and taking more dedicated anti-air.
War Walkers w/Scatterlaser, Eldar Missile Launcher (x1)AV10 105.33/HP; 880.80/EX
AV11 146.80/HP; 1452.82/EX
AV12 242.14/HP; 4144.60/EX
The Scatterlaser's high rate of fire and Laser Lock makes this option a little more practical against Flyers, as well as other targets. If you insist on taking Eldar Missile Launchers in your army, this is the least crazy way to do it, but that doesn't make it good.
3 Dark Reapers w/Exarch, Fast Shot, Night Vision, Aegis Defense Line, Icarus LascannonAV10 105.88/HP; 415.38/EX
AV11 133.33/HP; 540.00/EX
AV12 180.00/HP; 720.00/EX
These numbers are worse than they appear because if you bother to field this terrible AA squad, you'll probably bulk it up to have at least 5 Dark Reapers, which lowers its AA efficiency; frankly this squad is just anti-infantry with token AA thrown in. The Icarus Lascannon is a pathetic shadow of the Quad Gun, so add one to a unit of Dark Reapers only if your Quad Gun model got crushed underfoot or took a particularly disastrous tumble from tabletop to floor.
Warp SpidersAV10 85.50/HP; 684.00/EX
AV11 114.00/HP; 1026.00/EX
AV12 171.00/HP; 2052.00/EX
It turns out that, aside from AV14, there's nothing a Warp Spider can't kill. Although you won't be taking Warp Spiders as dedicated AA, they can make Flyer pay for showing up in their weapon range. Speaking of which, since Flyers tend to advance at least 18" onto the board, the Warp Spiders have a good chance to respond on the turn immediately following the Flyer's arrival.
3 Dark Reapers w/Exarch, Fast Shot, Night Vision, Eldar Missile Launcher, Flakk Missiles, Aegis Defense Line, Quad GunAV10 63.45/HP; 517.96/EX
AV11 86.33/HP; 810.00/EX
AV12 135.00/HP; 1620.00/EX
Like the Dark Reapers with the Icarus Lascannon above, the numbers for this squad are deceptive; this configuration is the minimum to get good AA in a Dark Reaper squad, but I don't expect to field it this small. I'd recommend adding at least 2 more Dark Reapers, which will boost the squad's firepower against non-vehicles and provide ablative wounds to protect the Exarch and Quad Gun operator. Adding those Reapers does dilute the AA efficiency, though, making this squad less effective against Flyers per point than the next entry...
Wave Serpent w/Twin-linked Scatterlasers, Shuriken Cannon, Holo-fieldAV10 78.77/HP; 671.58/EX
AV11 111.93/HP; 1159.72/EX
AV12 193.28/HP; 4245.23/EX
Who would have guessed that, barring re-rolls bestowed by a Farseer, the most efficient non-Flyer AA in the Eldar Codex would be its dedicated transport? These numbers assume that the Wave Serpent is firing its energy shield, but that's not much of a drawback considering that the transport will be attacking Flyers on the second or third turn. That means that the Wave Serpent would have already dropped off its passengers, and so no longer needs its shield to prevent Shaken and Stunned results that might ruin your disembarked unit's shooting. The fact that the shield's firing mode ignores cover is the icing on the cake when it comes to shooting down Flyers, and it is more efficient at killing infantry and light vehicles than the Fire Prism (and I had such high hopes for the Fire Prism...).
3 War Walkers w/2 Scatterlasers + Guide/PrescienceAV10 68.18/HP; 613.64/EX
AV11 102.27/HP; 1227.27/EX
AV12 204.55/HP; Glance Only/EX
These numbers assume a full squad of 10 Warp Spiders, plus 1/3 of a Farseer w/Eldar Jetbike and Singing Spear because I assume that the Farseer is using the other two powers elsewhere. It's not surprising that units with low BS, or are forced to fire Snap Shots, benefit the most from re-rolls.
Firestorm w/Shuriken CannonAV10 65.14/HP; 586.29/EX
AV11 97.71/HP; 1052.31/EX
AV12 195.43/HP; Glance Only/EX
Since the Firestorm is designed from the ground up to shoot down Flyers, it's no surprise that it makes such a strong showing. Unfortunately, shooting Flyers is the only thing it can do better than most units. War Walkers, Vypers, and especially Hornets produce far more mobile firepower against ground targets, a Wave Serpent is a much better transport, and it brings plenty of supplemental anti-air firepower to the table alongside the Crimson Hunter, the most efficient anti-air option in the Codex. Competition for Heavy Support slots also marginalizes the Firestorm.
10 Warp Spiders + Guide/PrescienceAV10 56.45/HP; 451.64/EX
AV11 75.27/HP; 677.45/EX
AV12 112.91/HP; 1354.91/EX
Similar to the War Walker example above, the numbers here are based on the 10-man squad with some of a Farseer's attention. The difference is that Warp Spiders are great on their own, whereas the War Walkers with 2 Scatterlasers are generally inferior to War Walkers with Scatterlaser + Starcannon.
Crimson HunterAV10 61.71/HP; 231.43/EX
AV11 67.50/HP; 270.00/EX
AV12 80.00/HP; 360.00/EX
You knew this one was coming. The Crimson Hunter is by far the most efficient tool the Eldar have to destroy AV12 Flyers. It's strengths and weaknesses are obvious, so I won't go into them in detail, but if you're in the habit of using them, an Autarch will go a long way to ensure that they get to apply their firepower where they need it, as well as survive long enough to do so.
Crimson Hunter Exarch w/2 BrightlancesAV10 54.00/HP; 208.29/EX
AV11 60.75/HP; 243.00/EX
AV12 72.00/HP; 324.00/EX
There's not much to add here; it's just a more efficient version of the Crimson Hunter.
Crimson Hunter Exarch w/2 StarcannonsAV10 41.99/HP; 171.53/EX
AV11 54.00/HP; 253.57/EX
AV12 79.35/HP; 648.00/EX
The Starcannon variant gets a 28.60% boost against AV10 and a 12.50% boost against AV11 while giving up 9.26% of its firepower against AV12; it also loses 60% of it's firepower against AV13 and 75% of its firepower against AV14. Long story short, if you're worried about Heldrakes, Vendettas, and Stormravens, Land Raiders, Leman Russ front armour, and Necrons in general, take the Brightlances, but otherwise the Starcannons will serve you very well indeed. Personally, I'd take the Brightlances just because the rest of the Eldar firepower can wreck AV10 and AV11 with ease.
That's all for this instalment, although I will happily edit this post to include your suggestions for good AA I might have missed, or to address in detail any special considerations that might arise. Have fun!