Eiryss2 is a Boss

After a couple of games with my Mercs after playing mostly Circle and Legion this year I'm realising how much I missed her. It's not just the upkeep removal she does so many things and I don't think a lot of new players see the full potential of her. A lot of times I see her run forwards, ping off an upkeep that's not particularly important and then die. I decided to try and write a primer on her for newer players or at least players who are new to her.

Read the Card

Almost every rule on Eiryss' card has the potential to win games and there are a lot of them. Lets go through her abilities.

Abilities
Stat Line - Mat 6 with two melee attacks. You should never be using them. Her DEF and RAT are massive and she actually has not terrible ARM taking into account those things. With ARM 12 and 5 boxes she can often survive unboosted blast damage. Really though you want to avoid even that. Lastly she is RNG 12 and SPD 7 so without any other range enhancers or movement shenanigans she can attack a target up to 19" away.

Advance Deployment - This can be a trap.

Fearless, Pathfinder, Stealth, Camouflage (increases her already ridiculous DEF even higher, she's DEF 20 in a forest, 22 behind a wall and you have to be within 5" to have the shot).

Arcane Interference - Her key ability. When she hits a model with a ranged attacks she removes - upkeep spells, animi and focus points. She also disrupts jacks she hits.

Sniper - She can do an auto point of damage and picks the columns that the damage goes into.

Technological Interference - This one can screw over bad players but is actually a great offensive ability. Non Myrmidon (Ret) Warjacks within 5" of Eiryss cannot be allocated focus or channel spells. That last one is particularly great when combined with:

Whiplash - If you miss her with a spell the model casting the spell is hit instead.

Deployment
First and most importantly advance deployment here is not a tool to get Eiryss further up the table. You should not be using Eiryss to buy table position or to vanguard your force. Instead use the advance deployment rule to place Eiryss away from your opponents best tools to remove her and across from the models she needs to affect.

Have a look at your opponents caster card. You'll generally see very obvious targets for their upkeep spells. Defenders Ward is probably going on Tiberion or the Avatar or maybe the unit of Cetrati, Admonition is going on your opponents most dangerous heavy (at least in the early game), Iron Flesh on those winter guard. Eiryss needs to be in a position where she can knock off the upkeep you are going to struggle the most with on the turn you it will matter the most. Or in a position where she can remove said upkeep and remain safe. Generally you want to look for terrain for her to hide in. Forests are ideal because they can completely block LOS to her and she can easily move through them to get LOS to the model she needs to shoot at. Buildings or other obstructions are the next best bet. You want to completely block LOS if you can because your opponent will put a lot of effort into removing her.

Importantly KEEP HER AWAY FROM YOUR JACKS. Remember she has a 5" bubble around her in which (unless you are playing Retribution) you cannot allocate focus to your warjacks.

Using Her Abilities

1. Remove enemy upkeeps on your own models. Crippling Grasp, Fear of God, Icy Grip, Blood Mark etc can be devastating however Eiryss can remove the upkeep allowing you to play normally. However be careful doing this to your warjacks because they will still suffer disruption. 
2. Remove enemy upkeeps on enemy models. The reason this is a seperate catagory is because to do this you will generally have to expose Eiryss. She can easily shoot the furthest back member of your unit to allow them to charge despite Fear of God but shooting even the closest enemy model in a unit will still generally mean putting her in a position where she may die. So before activating Eiryss ask yourself - Can she do this and stay safe, is she likely to be shot at or engaged and if so is the removal of this upkeep worth the risk of losing her. If not then keep her back and weight for a better opportunity. A lot of times there is one important thing Eiryss will need to do in a game, shooting a critical upkeep off a heavy/colossal, in those games she needs to stay safe but threatening right up until you are ready to deliver the killing blow. After which it is less important. You can lose Eiryss if it means your opponent loses their colossal.
3. Disrupt enemy jacks. Some of the best use I have gotten out of Eiryss has been in destroying one enemy heavy and then having her disrupt the other one to ensure my models remain safe. Again since you are shooting at an enemy model make sure that the risk is worth the reward.
4. Put her within 5" of multiple enemy jacks to deny focus. The no focus allocation bubble she produces is actually rather large. It's generally very difficult for your opponent to stop you from getting multiple jacks if they have them. This will almost certainly get Eiryss killed. It is however sometimes worth it. If your opponent is playing a high focus caster with a number of warjacks (or perhaps a colossal or two) then sometimes Eiryss can buy you almost a timewalk. 
5. Knock focus off a warcaster. This ability will win you games but generally only happens late game. This is one of her most powerful abilities and it requires you to keep her alive to the late game. It's one of the reasons that you should be playing Eiryss conservatively. Increasing worth late game is a theme with Eiryss, getting the most out of her means playing her very conservatively until you hit the point where she can do the most damage. Watch out for shield guards or sac pawn and again don't waste the ability if that's the only attack you are getting on the caster. Use it at the start of an assassination run.
6. As a scenario piece. If you are playing her right she will often be alive at the later stages of a game. This is important because this is when the scenario very strongly comes into play. Eiryss is DEF 16 which makes her very difficult to remove and SPD 7 and has pathfinder and stealth. There are a large number of games where I have put Eiryss into the zone late game and my opponent has been unable to spare the attacks to remove her. Tips for this run, get her into your opponents back arcs (no aiming and less models can reach her than if they could charge). Abuse terrain in the zone, abuse stealth and whiplash by staying as far as you can from enemy models. She is also very good at clearing that last enemy model from the zone. 
7. Abuse Sniper. She can pick columns and do an auto point of damage. I almost didn't put this one on here but it is something I do fairly often. Often you will find yourself leaving a model with 1 point left either in total or in a vital system, generally this is a larger base so Eiryss can shoot at it from behind a wall of your own models and her RAT is high enough that she often does not care about shooting into combat. That or she can pick out a model in shield wall (or just kill an important high DEF solo, though that's her RAT not sniper). She can kill the last model contesting your zone or the only model your opponent has B2B with their flag. This is the crappiest use you will get out of Eiryss. Don't do it unless you are confident you can do it and be safe. 

Playing Against Her

Get her off the table as soon as possible. She becomes more and more powerful as the game progresses. This is often easier said than done. Shooting AOEs at her is a good start, you'll miss (stealth) but 1/3 she gets caught in the scatter and you just boost blast damage. Magic Bullet and other abilities that jump or auto hit can be useful but they require your opponent to put her in the game. They do nothing if she is standing behind a forest just outside of your opponents deployment zone. If I have the totem hunter in my list killing her becomes his purpose in life. Sometimes though you just have to suck it up and wait for her to pop out.

Do not shoot spells at her. With stealth and whiplash and her high DEF there is a super high chance that spell is coming back into your casters face. Don't send jacks after her. Be aware of her DEF, if you run to engage her do it with something that has a danger of killing her with the free strike. One MAT 5 or 6 model engaging her is not going to cut it. Even if your opponent can't free up something else in their entire army to kill that one guy she will generally survive the free strike.

Bait her out. Put a juicy upkeep in her range but out of range of the rest of most other things. I have for example run a unit of bloodtrackers with regrowth on them straight at her. It hurt to recast it but I killed Eiryss and lost only 2 bloodtrackers that turn. Had my opponent held her back and waited until he could have killed half the unit that would have been much more painful.

Don't forget where she is. Eiryss will spend most the game hiding behind a terrain feature waiting for her opportunity. Often this means you can't physically see her. Make a note or mark it if you think it will be a problem. It's a mistake I've seen very good players make so don't be afraid to put something down on the table so you remember.

I think that's it. The most important thing with using any piece is just to play it. Put her on the table, get a feel for using her and the different styles of using her. Play against her and see what is the most effective way to remove her and see where she is the most help and where she causes the most problem for your opponent (or for you for that matter).