Uxía comes in two versions that will promote a somewhat different playstile.
In her SAS CP version, Uxia comes equipped with a boarding shotgun and grenades, as well as smoke grenades. She also has an AP CCW, MSV1 and comes with Infiltration and Camouflage.
The covert action version (and you will VERY likely use that one) keeps her shotgun but loses the frag grenades and MSV1. She is also downgraded to Limited Camouflage. The really important things are the ones she gains though: D-Charges, Specialist Operative, Superior Infiltration and 2 assault pistols!
Both versions also have Kinematika L1, Multiterrain and Martial Arts L2 as well as Fury: Frenzy.
When it comes to the SAS CP version, things are explained rather quickly: This is basically a normal SAS loadout with some tweaks. Since there is no specialist skill here, Uxía will be used as an assassin unit most of the time. She will do ok at this, but her Camouflage is largely offset by the Fury characteristic, meaning that she will likely be running around with Mimetism most of the time.
The real kicker however is the covert action style Uxía - welcome to a hot contestant for best specialist unit in the game!
With this loadout, she does almost everything better than her earlier verision and then some! She can simply shower enemies with bullets using the 5 dice of her 2 assault pistols, while she stays a threat to groups with her shotgun. But the most important thing here is the utility that she gains via Specialist Operative and d-charges. If you have the corresponding secret objective, Uxía is an almost guaranteed ITS-point. Her smoke grenades are now also great for covering an objective she can then take herself!
Whether you want something gone or an objective in the midfield, Uxía is your woman for the job!
Both versions can make good use of Camouflage and smoke grenades, especially with a physique of 13! While the SAS CP Uxía also profits from Mimetism, the covert action one has to make do without it.
The lack of any armour or BTS is a sad thing indeed, but shouldn't come as a surprise for Ariadna skirmishers.
Uxía is pretty much a walking toolbox of sorts. If you have the "Sabotage" secret objective and the first turn, you have a very easy ITS point by either deploying in midfield or even using superior infiltration to go beyond.
Because of her ability to deploy midfield and remain in a Camouflage marker, Uxia excels at 'chicken' play against other forward deployed skirmishers. By moving towards an opponent's non-marker state model with the intent of moving behind them to surprise shot from the rear, she places them in a tough situation of either Discovering and being shot unopposed or allowing her to move behind them to fire her boarding shotgun with the only response being a Dodge at -3.
The SAS CP version of Uxia has Grenades, and with the ability to Infiltrate up to your opponent's deployment zone, if you can find a very safe place for her and make the roll Uxia can spend orders to Speculative Fire those grenades onto any large cluster of opponents. While she'll be throwing at PH -6, she'll gain +3 from range if they're within 8 inches and be throwing those grenades on 10s-- a very respectable number.
If you plan on really burning some orders, failed Guts rolls by your opponent can be used to push the targets of the speculative fire in any direction you choose thanks to the loose targeting of the template-- for example, if a strong ARO piece is in cover and you'd like to push them out of it, smart placement, a good roll and a bad roll by them can push them out of the template and out of cover. Alternatively, you can use Spec Fire to try to pare down a fireteam by "pushing" the link leader away with a failed Guts roll but this is far less likely unless your opponent has the coherency of the unit spread very very thin.
Also keep in mind that you can actually use d-charges in CC! With MA L2 and CC21, you can be a very nasty surprise against TAGs - especially when you use surprise attack.
As soon as Uxía is discovered, it should be relatively easy to counter her - preferably with longer ranged weaponry. If you face the covert action version, she doesn't even have Mimetism. Keep in mind though that she has smoke grenades, so a smoke dodge at PH13+3 can be very effective.
Uxia McNeill is dead meat against any long-range ARO pieces with MSV2, as well as twin templates due to her low ARM.
Covert Action Uxía fits basically any Ariadna list and is actually my absolute go-to specialist. The premium in points, compared to the most basic version of a midfield specialist (Foxtrot Ranger Forward Observer), is well worth it for the smoke grenades, d-charges and other weaponry.
Depending on what secret objectives you drew and if you have the first turn, Uxía works best when deployed in midfield. If you don't have the first turn, hiding her (e.g. in a buidling at 2nd level) might not be a bad idea. Keep in mind that she has smoke, so if you have an objective out in the open and no enemy MSV2 to guard it, she is the one that can get it for you.
If the opportunity presents itself, you can also use Superior Infiltration to go beyond the midfield at PH16 - but I rarely ever do that. Simply put: The chance of saving a few orders that way is not compensating me for potentially loosing such an important player in the midfield.
Active Turn Role
Uxía will take objectives and surprise enemy troops with a hailstorm of bullets from her two assault pistols. If you get the opportunity, you can also give clustered enemies a hard time with the shotgun.
Uxia McNeill pairs very well with McMurrough. Placing Uxia in McMurrough's warpath, prone, behind cover allows her to throw a smoke grenade unopposed in order to cover his advance, and once McMurrough's orders are expired (or he is!), Uxia can take advantage of his smoke grenades to move forward and engage or take objectives at her leisure.
If you cannot hide her properly, Uxía can still perform a very good smoke dodge. If you place her in a cramped environment (e.g. a building) she can also make good use of her powerful boarding shotgun. Finally, Uxia has a solid option in her SAS CP profile of AROing with a grenade to strike units around corners that don't expect a circular template to be laid down on their active turn, and the difference in numbers between the boarding shotgun and grenades are minimal (BS11+6 to 17, PH13+3 to 16).