General Responses to Stealth: These are things that are more universal and will work against any flavor. The biggest is melee. Finding things in Ret that can't fight is actually pretty hard, so even by accident you should have units that can throw down in your list. With CMA and a 9.5" threat range especially if opponents are trying to run to engage them and keep them from shooting, the MHSF can fight suprisingly hard. Things with Reach are also good because they can engage multiple models more easily to keep them from going anywhere while prepping their counter attack. Sprays always ignore Stealth making Discordia invaluable if you're good enough at maneuvering to not have her just get eaten after 1 spray and Dhalia Hallyr and her pet also fall into this category although Skarath has less POW and RAT (and doesn't fight as hard). In general most Ret casters can expand jack threat range and most Stealthed heavies don't have ways to buff threat range, so a simple solution to melee heavies outside of specific cases like Shadowpack where they can threat far is just to have your heavy beat their's face in using Mobility, Locomotion, Telekenesis, Mirage, Whip Snap, etc. Another response is simply walking into stealth range to shoot. SPD 6 units still threat 11" against stealthed targets which is enough that it's going to be rough avoiding this especially if you're carpet bombing or spraying units to death on the way in and by doing so, increasing the importance of them closing fast and not trying to play the threat range juggling game.
High Def, Low Arm Single Wound Infantry: This is probably the easiest and most common type of Stealth you'll have to deal with. It encompases Striders, Kayazy Assassins, Hex Hunters, Bloodrunners, Bloodtrackers and basically everything in the ARM 11-12 category. ARM 13 can be dealt with with similar responses, but can require above average dice rolls. As a note, most things that will deal with this flavor of Stealth easily, can also deal with things like Nyss Hunters, Daughters of the Flame and Gun Mages exceptionally well. The best response here is Stormfalls. When the unit doesn't have to worry about direct hitting, having two units carpet bomb an area with 8 3" AoE's from 16" away tends to kill everything including local wildlife. One Response that's great for any infantry, but especially good agains the high DEF ones is the Phoenix. Without exception, nothing here wants to pick a fight with the big guy because he'll easily have enough health left for a combustion even after they charge, so going back to the afforementioned ways to buff a jacks threat range, run a Phoenix on that flank and dare them to come at you. The worst strategy here is usually "Moar Sentinels" as it was so aptly put because anyone who can read a stat card can see Sentinels are going to have problems agains the DEF numbers these units can put out since they lack any way to buff attack rolls.
High ARM low DEF single wound infantry: These tend to be harder to Respond to, but in this case a good melee unit like Sentinels in a properly spaced formation to take advantage of Vengence is a pretty good Response. As stated previously, the Phoenix works against any and all infantry types, just try not to get him charged by Bane thralls and you're in business. Generally though, low DEF infantry are much easier to respond to because even ranged units like Invictors or the MHSF can tear them up with some very average dice rolls in melee so even if they get to you, some good spacing and use of your resources can make these units little more than a minor annoyance. Another strategy is using Aiyana. If she Harms a unit, Stormfalls are effectively throwing POW 8 blast damage, which is enough to hurt a lot of ARM numbers with only average or slightly above average dice.
Solos: Discordia, Discordia and Discordia. She threats 16" with her spray not counting the myriad of ways Ret has to get her there faster. Give her two focus (one if Ravyn is feating) and that problem solo is not a problem. On average dice she's got good odds of bringing down solos with good ARM like Tartarus (roughly a 50/50). MHAs are also a solid choice, though not neccessarily for the reasons you're thinking. Yes they can certainly roll up on an enemy solo from 14" out and one shot most of them, but they can also run 14" and engage them. Nobody ever takes free strikes from that model. Then Rahn can run an Arc Node up and cast if he's that desperate or Kaelyssa can just shoot them.
Casters: Stealthed casters aren't as much of a problem as people proclaim. It forces you into attrition, but if you're playing Retribution right, attrition isn't a problem because you're units are generally better and will hold their own. While it's annoying you can't end-run the game as easily, it not an auto-lose or even an uphill battle. Just wipe out their army and let them be Stealthy all they like as your caster/jack/unit/solo beats their face in.
Heavies: Sentinels and your heavies are the best responses here. Sentinels have the numbers and Vengence making them hard for heavies to deal with if you just run two models up to engage. Also, back to the beginning, I mentioned Ret threat ranges. They're usually better than enemy heavy threat ranges. The last time I had a Stalker Prowling on the edge of a forest, I locomotioned a 3 focus Discordia at it and she beat the thing senseless. I've also had suprising luck using stormfalls to walk into Stealth range and hit the target with Brutal shots and the MHSF walking up to jacks to unload POW 10 jack hunter shots at the target although this is a good bit more luck based. Skereth Issyen is another solid choice in this category along with MHAs, since both can do massive damage from a long distance away. Incidentally, neither are particularly good at dealing with other flavors of Stealth with the notable exceptions of casters and, in the case of MHA's, solos since they lack the attacks they would need to do serious damage to a unit in the limited time before the enemy kills them and trading a 5pt Dragoon to kill a 2-3pt solo puts you on the bad side of a trade.
--The best response to eLylyth is a blitz. Her list is usually light on shots that if you just run forward with an infantry swarm she'll have some tough Decisions. Don't use Shieldwall, Ravagores can cause continous fire and I've done that enough with Stormfalls and Star Strikes to know it's a death sentence for the unit. The MHSF is an oddly good choice because only Lylyth and her battlegroup ignore stealth and the MHSF with CMA fight a lot harder than striders and Deathstalkers. Combine that with AD and Pathfinder and the unit can actually maul striders suprisingly well. Your biggest problem in this matchup is acutally preventing the assassination, but eLylyth doesn't have much to break ARM, so a good camp with Discordia humming in the background can go a long way to shutting her ranged assassinations down. Vyros and Rahn do it best. In general, the strategy here is to engage and stay engaged as early and often as possible with Reach infantry (halberdiers work best because they're cheap) and the MHSF while keeping your caster from assassination.
--eKaya is a different animal. Her beasts threat far and she's got a turn on her feat of being able to pound you without resprisals. Oddly enough though, she ends up being less threatening than eLylyth in many cases because a good Reach screen will stop beasts. Garryth's feat also turns off the return part of eKaya's feat, so if you know what you're doing and go first, it's suprsingly easy to be on top of the army before they know it and if eKaya sends her beasts in, she can't stop them from getting murdered for their efforts. Like Lylyth, the key here is to apply pressure, but because the only threat is melee in nature, it's easier to throw up barriers like a Reach unit with Polarity Shield or Kaelyssa's feat to reduce their threat range to a more managable level.
--In regards to both armies though, they need fury management solos or they're going to have serious problems and the fury management models aren't Stealthed, so the MHSF can make some serious money just hunting down the 2-3 models the army needs to not fall on it's face. Neither matcup is easy and the better the opponent, the less your odds of success, but with a good use of forces, you can at least make it real game.