All profiles of the Samaritan have Vorpal CC weapons and Flash Pulse allowing them to be extremely dangerous to multi wound models within short ranges and defend themselves from longer range attacks in ARO. Beyond the usual pistol they have options for Breaker Combi rifles, Spitfire at a reduced SWC to the normal cost, and Plasma Carbines. The last option also comes with an EI Assault Hacking device, making for a deadly specialist option.
Samaritans do not have Martial Arts, as one would expect, but instead come with the Protheion rule giving them bonuses in CC and the ability to gain bonus wounds from damage inflicted in close combat or via Coup De Gra.
Standard Medium infantry ARM of 2 and BTS 3 means the Samaritan has a better chance against small arms. V: No Wound Incapacitation means they can remain active while unconscious allowing them to stay in the fight. This interacts with Protheion allowing them to "heal" wounds suffered so long as there is an enemy near by to eat.
Beyond this they have Hyperdynamics L1 and Kinematica L2 meaning they will dodge on 15 and move 4" making this a tempting option for them. Remember that Super Jump allows them to make this movement vertically and fall up to 4 inches without suffering damage, which allows for some unique dodges. Flash Pulse on all profiles means that from medium ranges they can also shut down a shooter, though they are often better off just dodging.
The Samaritan's Vorpal CC is tempting to use, but often it is better to rely on their other weapons to leave an enemy model unconscious so that you can kill it with Protheion and get a bonus wound.
Super Jump and dodge means you can dodge in any direction, including Vertically. You can also "vault" while super jumping allowing for a little extra movement, or fall up to 4" without penalty. This combined with Kinematica means the Samaritan can do some truly weird dodge movements.
Note that all the above also applies to Engage.
Consider using the Samaritan later in the turn or even on turn 2 or 3 and setting up a buffet of wounded troopers for it to eat with Protheion. Once powered up it becomes a much more formidable model.
Killing the Samaritan before it gets fed is essential. It should be considered a higher priority target both because of its expense and the potential for it to snowball. This is especially true if it is leading a Harris of TAGs in the Onyx sectorial army.
Shock weapons are a natural counter to the Samaritan, and engaging it at range is ideal. The Samaritan will often dodge if it is not in a link, so understand that you may be granting it free movement or only get one shot at it. Another good cure for the Samaritan is placing a mine near it, then attacking with other models to force it to dodge the mine.
If in a Rodok link team, look to remove at least one of the Rodoks first to lower the Link's bonuses. They will be easier to kill than the Samaritan most of the time.
Once a Samaritan gets fed, it becomes immune to Shock weapons, so the best plan is to kill it early or, if necessary, deny it free wounds. This is easier said than done, but models with Dogged can choose to use it so as to die before the Samaritan can eat them. Likewise it is better to have a model fail a Doctor roll of Medikit PH than leave it on the board near the Samaritan.
Finally, don't consider CC totally off the table. The Samaritan is not as good as a Legate in close combat, and will likely use it's Vorpal weapon which will either not wound or kill outright not leaving a corpse to feed on.
The Samaritan requires planning and experience to use well. It's pretty easy to lose it early and feel like the model isn't worth it's points, but properly supported the Samaritan can be very effective. Think of it as an investment. If the Samaritan gets a chance to feed, it will become far more powerful than it's cost.
In the Onyx sectorial the Samaritan is a critical addition to a Rodok link, adding some much needed CC and feeding of the link's kills. Unlike the Umbra Legats, it is also capible of moving with them thanks to Super Jump.
Keep the Umbra Samaritan far from harm's way, in a position to move up the table. It is worth moving it up the table slowly if the path is safe, and worth leaving it behind safety until it has a chance to feed.
Active Turn Role
The Samaritan wants to look to disable or drain low stat models with the help of a link or the support of the rest of the force. Once fed, or if given a clean shot, it can use it's Vorpal weapon to kill high value targets. While the Legate is strong early game where it's base line stats and forward deployment let it start in a good position, the Samaritan is good late game once it has enough wounds to try risky shots.
The Samaritan dodges very will with Hyperdynamics, and Kinematica L2 allowing it to get to safety when under fire. Flash pulse and Wip 14 means that it can blind ramboing troops dead, and V: NWI means it has a back up plan if it fails. In a link it will almost always want to Flash Pulse. Alone it is often better off dodging unless the enemy is in the open and without defensive tech like camoflauge.