With the correct selection of skills, weapons and mods, the Infiltrator not only becomes the most lethal sniper in the game but possibly the most lethal class in general, based mainly on how quickly they can make groups of things they don't like become dead and simultaneously rarely receive damage. The Soldier can be focused to be a decent sniper, it's just that the Infiltrator will be a lot better at it, and every other class and build have key aspects on how they fight that a sniping Infiltrator can surpass.
- In mid to long range weapon combat without sniper rifles, other classes with other weapons need to fire multiple shots into any target, even weak ones, in order to kill them. At these ranges they often can't get the critical headshots effectively, and even if they do they don't have as many bonuses to damage on those as the Infiltrator does, and they have to remain in the open exposed to fire for set durations (as does the enemy they are targeting). An Infiltrator, on the other hand, goes see enemy-bang-move on, concentrating another class's extended burst of fire into one fast shot focused into the critical area of the target, which also drastically reduces how long they have to be exposed to fire even if they don't use Tactical Cloak. If the other classes do use a Sniper Rifle they won't be anywhere near as good as the Infiltrator, both for not having as much time dilation in the scoped view and less damage per shot.
- In short range weapon or melee combat, the player is always dangerously exposed to enemy fire. Some classes can still handle this fairly well, but getting in range of enemies to constantly do it can be difficult and time consuming. For an Infiltrator, not only can very few enemies even get into close range against them alive, when they do have to do short range combat (probably with a shotgun), Tactical Cloak makes it very easy for them to safely position themselves for an ambush or simply back off to get range.
- In power or tech-based combat, this not only isn't very good at very-long-range combat but also depends on distinct power combos, possibly involving your squadmates as well. Only the weakest enemies can be defeated with a single power, even on normal difficulty, and using multiple powers per enemy takes more time to get through them plus increases how long you have to be exposed out of cover. A sniping Infiltrator will simply tear through them faster with less complications, and honestly doesn't care what his squadmates are doing besides distracting the enemy; they're carrying everything they need on themselves already.
In short, the Infiltrator will kill enemies faster and stay protected behind cover more than anyone else, if built properly. How to do that build will be discussed below.
The most lethal sniping Infiltrator should devote themselves to maximizing the effectiveness of their sniper rifle against all possible enemies and defenses, and should not care about extra status effect powers, disruption, etc...if you can kill them in one shot when they are at pristine health, who cares about status effects and disruption? The choice of power builds shown below follow this mentality.
- Cryo Ammo, Sticky Grenade, ignore them entirely. You have a bunch of better options.
- Sabotage, at most get 1 level so you can use it for the odd time you might want to distract a group of enemies by a turret. Or just shoot them.
- Incinerate, perhaps get up to 4 levels with the Radius option, it can be a useful emergency backup and good for taking out the little rachni swarmers. If you eventually go further, Recharge Speed and Armor Damage are good options for ranks 5 and 6.
- Operational Mastery, definitely upgrade this quickly. You want Weapon Damage at rank 4, Weight Capacity at rank 5 and Sniper Damage at rank 6.
- Tactical Cloak is your key skill for positioning and putting a heavy hit on a big target, so upgrade it quickly. You want Damage Bonus at rank 4, Recharge Speed at rank 5 and Sniper Rifle Damage at rank 6.
- Disruptor Ammo, with its new bonus to Barriers is going to be your bread-and-butter ammo power against many enemies. Upgrade it quickly. You want Damage at rank 4, Headshots at rank 5 and Damage at rank 6.
- Armor-Piercing Ammo is your most effective choice for your bonus power once you unlock it by regularly talking to Garrus. It lets you free up a mod slot on your sniper rifle and lets you use it for the key (and rare) situations where you have to hit an enemy through cover (instead of one of their often-exposed body parts) and heavier damage against bosses. You don't need to max it very early but you eventually want Damage at rank 4, Headshots at rank 5 and Pierce at rank 6.
- Fitness, although definitely useful isn't too critical to max out early since you keep behind cover and/or cloak frequently. Ignore all the melee bonuses, you want Durability at rank 4, Shield Recharge at rank 5 and Durability at rank 6.
Combat with this build is fairly straightforward. See the enemy, check that you have the right choice between Disruptor or Armor Piercing Ammo, Cloak if they're tough or you need protection from fire, and drop them plus potentially two of their nearby friends with the Black Widow. Repeat.
For its brutal combination of hitting power, clip capacity and ammo capacity there really is no question, a sniping Infiltrator is the most lethal with the Black Widow upgraded to rank V, with the Concentration Mod and Extended Barrel equipped. With the power build listed above and that weapon you will drop enemies of every known type, including groups of weaker ones or the big tough ones and bosses, faster than any class or squadmate of any build can match. Therefore your priority from when the game starts is to get this weapon as quickly as possible from Spectre Requisitions, at 250000 credits, but you also can't hamstring your gameplay up to the point that you get it. The best way around this is the following:
- Equip and upgrade the M-92 Mantis to rank V, with the Extended Barrel and Concentration mods.
- Equip and upgrade the M-23 Katana to rank V with pure damage mods to handle close range combat. You may decide never to switch off of it.
- Don't equip any other weapon type, and don't upgrade any other weapon at all even for your squadmates (any squadmates that can use sniper rifles and shotguns can use these two you just maxed out).
- Don't buy ANYTHING else, at all, from any store anywhere on the Citadel or the terminal in the Normandy. There is nothing else critically important at that time, even items for quests, that can't wait until after you have bought and maxed the Black Widow.
- When doing missions always go over every area with a fine-tooth comb. One good method: when using the guide navigator (V by default) to find the next primary location, go everywhere that you can access that isn't at that location and search them all.
- On the map, always leave the Priority missions to last, they are the main campaign ones. Explore every system with Reapers at them and max out their collectibles. One key note on this, virtually every solar system you will be exploring like this has a shipwreck in space with fuel for pickup. Carefully plan out your use of these so you have to buy as little fuel as possible from Fuel Depots; for example, don't pick up the fuel collectible from the system with the Mass Relay until you've finished every other system in that sector, to refuel yourself from the trip back at least partially for free.
For squad members in general, you'll kill enemies far, far faster than any other squad member even at max skills can do. Your best choices are tough members that are effective at distracting the enemy for a long time as well as doing decent damage. Garrus is an excellent choice because he can use the same upgraded sniper rifles as you, and the ultra tough James Vega who can use your upgraded shotgun and disruptive Incendiary Ammo with an assault rifle is another excellent choice.