Sin Eaters are dedicated ARO pieces which trade their burst value on active turn for their enhanced burst on ARO. As such they are probably the last troop you want to send in to hunt enemies.
Sin Eaters are completely designed for defense. However as the saying goes, attack is the best defense. Their Neurocinetics skill allows them to shoot their full burst in ARO, thus reducing the chances of being hit and also increasing substantially your chances of scoring damage or even criticals on attacking opponents.
Sin Eaters also benefit from Mimetism, so when behind cover they impose a -6 penalty to attacking BS attacks. With their armor of 3 they can substain some punishment as long as they are hugging cover, and Religious Troop will help them hold the line even when they are hit with enemy fire.
There is little to no subtlety on the Sin Eater. He's meant to be the unmovable wall against which your opponent will break into. The most important thing to take into account when using him is deploying him in an optimal position where he can maximize his chances of hitting, ideally negating the enemy the chance to hit him from his weakest range band.
Having a medic or a zondbot nearby to heal him when he goes down can give you a second chance if he ever falls to enemy fire.
Remember Sin Eaters are also armed with pistols, and they also benefit from the BS bonus in ARO from Neurocinetics, so if someone gets into your close range and you are armed with a long range weapon like an HMG or Sniper Rifle, you could always bring out your handy pistol to take out the attacker.
Sin Eaters are tipically deployed behind cover, and have a Mimetism penalty to be hit with BS attack so the best approach to dislodge them is using visors or troops with Marksmanship.
Direct template weapons particularly on speculative fire can help to bring down the Sin Eater without facing his ARO burst.
Remember you cannot divide burst on ARO, so using coordinated orders can force the sin eater to choose between several objectives, and give you a chance to score a clean unnoposed hit on him.
As ARO pieces, Sin Eaters will rarely be using the order they provide to the pool so they are extremely useful cheerleaders, specially in Bakunin. They sinergize well with medics and zondbots who can bring them back for a second chance at denying enemy advance.
In list composition, the usual rival for his spot is the Total Reaction bot at 26 points and 1 SWP. Starting at 30pts the Sin Eater is a tad more expensive but benefits from higher BS at 13, and Mimetism. He is also infantry so he can dodge without penalties if needed. On the other hand the Sin Eater can't benefit from Marksmansip hacking programs.
When choosing a Sin Eater, it's important to take into account the range you want to prottect to give him the optimal tool for his job. If you plan to defend close objectives, prefer the Spitfire, or choose the HMG if you pretend to deal with long range threats. The MK12 is a cheap option for medium range with no SWC cost that can provide reliable damage.
While the Sin Eater has a Multi Sniper Rifle profile, is the one that benefits the least from his neurocinetics, obtaining only Burst 2 in ARO. As such you could probably find more cost effective B2 sniper options on fireteams.
On generic you can only field a single Sin Eater so the choice of weaponry becomes more critical. On Bakunin though, fielding up to 3 Sin Eaters allows you to cover all your possible ranges with deadly ARO burst fire.
Deployment is critical when playing a Sin Eater. You want to maximize the amount of area that he can threaten whili minimizing the area where he can be made most vulnerable. That means knowing precisely where the range bonus of your weapons lie and the avenues of attack the enemy can use to outmaneuver you.
If you are armed with long range weaponry like HMGs or Snipers, you want to be on a high spot that gives you LoS to most of the board. Be sure to hug cover, as you are still a 1 wound trooper. If you are using short range options like the Mk12 or the spitfire you would be better covering enclosed areas with tall cover where enemy long range weapons can not target you, and anyone who enters the area would be on your preferred range band.
If you deploy more than one Sin Eater, try to place them so their optimum range can cover the bad range band of the other one so the enemy has never an easy route of approach. Also place your Sin Eaters in a way that they can cover both sides of the board so any drop troop will have to face their extra burst when deploying.
Remember to keep a medic near your sin eaters, even though they are durable, they can go down, gut giving them a second chance at shooting things might be just enough to make your opponents day miserable.
Active Turn Role
Sin Eaters are not meant to be moved, specially not on Active turn when they are the weakest. The only reason you would want to move them is to find a better position to threat the advance of the enemy, and even then you need to be sure to not cross any enemy line of fire while rellocating.
This also applies when getting up after being healed from unconscious state. Be sure to be out of sight when taking your new position or you might end up depending on a single BS roll to confront any AROs coming your way.
As previously stated, playing Sin Eaters in 95% about proper deployment. As such, when you choose your weapon loadout you need to be sure what functions you plan them to be performing on the battlefield. For that, knowing the mission is the first thing you need to take into account.
If you need to take an enemy near the enemy lines and want your Sin Eater to provide cover fire for any enemy trying to outmaneuver you, favour the long or extreme weapon ranges of the HMG and the sniper. If you plan to defend your deployment zone or mission objectives nearby, an MK12 or a Spitfire are your to-go weapon options.
Once the battle starts, rarely you would want to declare any ARO aside from shooting. Even if you are put at a -12 BS penalty from stacking odds, rolling 3 or 4 die gives you a higher chance to score a critical and survive than actually trying to dodge the incoming fire.