reading through the 2nd battle report more closely I think I can identify a couple of things that could have been done better (No insult intended at all, your tactics were standard for tau doctrine, but you were fighting a very non-standard foe in the tricksy eldar.)
First off was hammerheads with submuntions are certainly viable, and the submuntion shot is "good enough" compaired to the ion overcharge (and doesnt get hot). However, you used them against reavers, but due to the large size of bikes, even large blasts only hit a couple of them with each shot. You should have pieplated a unit that was more densly packed, like his warriors. don't forget that st 6 wounds ignore his feel no pain.
Secondly was that outflanking your foe with harrassing units is a excellent idea in general - just not against DE. A unit like your stealthsuits will do a little bit of damage but in the end be fairly easy for them to kill and might just provide them with a pain tolken. If you are going to use a stealth team then Id suggest using them as backfield armor hunters once his warriors are forward - but even then you'll only get one shot as the ravagers can outpace your suits, and those reavers can hit you anywhere within 48" so they can out-rapid-response you.
Some of those players made excellent points deconstructing our weaknesses. "Tau are only slightly more mobile than guard" "Moving 12" then shooting makes all the diffrence [compaired to JSJ's 6" then shoot], and "Markerlights being able to buff snapshot can make for some unexpected maneuvers" are lines that caught my eye.
He's a canny dark eldar player, who is not afraid to go to ground. I can see why you wanted a lot of ignores cover weapons. Im not certain that bladevaneing a unit off the table merits a pain tolken, Id have to look up the FAQ's for that one. don't be afraid to go to ground yourself - if it forces him to spend the time to shoot it with more units or to charge it that slows down that unit even more (hopefully). He was right that having special weapons sniped out isnt as big of a downer for DE as it can be for other races (a splinter cannon is just a couple more shots after all) so if you have snipers id look to kill blasters and, very importantly, the squad leader, if one is present. In his list, that would have been the arena champion, who's melee ability alone is as good as the rest of his squad's combined. Other dark eldar armies, the grenades are from the squad leader - kill the squad leader, the squad looses its grenades.
Specifically against that list - don't be afraid to tank shock his warriors out of position/cover. It looks like he is reliant on his backfield lances and forward blasters to get his hit, he's not going to haywire you as I don't see one in that whole list (unless that list is incomplete for wargear) and pushing him out of cover could be a death sentance for his troops. You'll risk a death or glory from the blaster but situationally it could be worth it.
I've made a big deal about killing off all the raiders - thats because a warrior squad without a raider is NOT a mobile squad anymore (fleet on a shooty unit isnt a great advantage). If you don't kill even the empty raiders you can be suprised how fast the DE can redeploy.
I think that this specific dark eldar player is going to try to kill your markerlights asap, he seems quite concerned about them in the writeup (especially with supporting fire). Use that as bait - I remember there was a BA player that I used to play that I knew, no matter what, was going to try to charge his bikes into my broadsides every single game. Predictability is death
The key to his list seems to be the reavers. Im suprised he didnt take caltrops on them for all the bladevaneing he did, but that is still a lot of reavers. With blasters, he'll only be closing to 18" before jetting away rather than the 9" his heat lance's would have required, so if he gets closer than that he's thinking about charging something. If possible, charge them, that will deny them two attacks per model (one for hammer of wrath, and the +1 a for chargeing) and while you may loose, you should kill some of them. Kroot would be nice here, as their ap 5 would still be effective. Just be careful about charging them based on the combat drug effect - if he gets initiative or weapon skill or run speed it wont matter much but the +1 a or +1 s rolls could hurt.
Im thinking that a crisis team with missilepod/flamers might be useful against him. flamers are scary to dark eldar, as you could jump out and toast a whole squad of warriors in cover, or wall of death anything that tries to assault your units. When not in flamer range the missile pods are more st7, and for support systems you could either buy them drones with a drone controller for more markerlights, VRT's so they can hit and run out of any assaults, or stims for more durability (as they will be a fairly high priority target for him, but watch out for st 8 lances/blasters ignoring stims). Or maybe just twin link the flamer. Three of those (before drones) will run you less than one of the riptides. perhaps two squads of two?