Sniping
In combat, aiming only represents lining up a shot for a few seconds before squeezing the trigger... and that's fine for combat purposes... but it doesn't cover what a sniper might well want or need to do.
Patience is key - you wait for the ideal moment to shoot. So, require Awareness tests every minute in order to spot his moment. Alter the difficulty as appropriate for the task and the distances involved.

Every successful Awareness test adds PB+Degrees of Success to the sniper's "Patience score". Failure reduces it by 1 for every Degree of Failure. When this score reaches a number determined by the GM, the sniper can take his shot. He spends a full-round action aiming and makes an attack roll, modified as appropriate for range, visibility, placed shots and other factors, including a +30 for the target not being aware. If the attack hits, roll for damage as normal, but ignore the targets Wounds score - the hit is an immediate Critical.

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There - a system loosely based on the Investigation Skills rules already in the DH rulebook, that allows snipers out of combat to get their one-hit-kills. It'll still take a good weapon (preferably an Accurate one) and an appropriate selection of talents to make the most out of such shots (Crack Shot, Mighty Shot and Sharpshooter are ideal here to ensure you deal as much damage as possible for that single shot)

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"Sneak Attacks"
This is a bit trickier. Again, it's not suitable for combat - the hectic surroundings and generally on-edge enemies make it functionally impossible.

For this, you'll need... stealth skills (Shadowing, Move Silently and/or Concealment), preferably mastered to some degree and/or with equipment to make them easier still to use... and some way of attacking in melee.

To deal a single lethal blow, you need to catch your target completely unaware - if you're noticed, you'll have to duck from view and start again at best, and fight for your life at worst. You also need to be so close to him that you can smell his last breath as it escapes from his mouth or mangled throat.

To approach that close, you need to make appropriate Stealth tests. For the most part, Silent Move or Shadowing are most useful - Concealment is only really relevant if they're looking in your direction at the time, or if you need to duck out of sight quickly to avoid their gaze. The difficulty of this test/these tests vary based on the surroundings (it's harder to sneak on gravel than on soft carpet, harder to hide under bright lights than in the gloom, and easier to shadow someone in a dense crowd than in an open field), and the target's opposed Awareness test is modified by his general alertness (if he's tired, bored or generally indifferent to the proceedings, he'll suffer a penalty... if he's paranoid, wary or suspicious, he'll get a bonus).

Attempt a stealth test (opposed by the target's awareness) every round. If you succeed and your target fails, you get a number of metres closer to your target equal to your Agility Bonus (if both of you are moving, you close the gap between you and your target by that many metres; Unnatural Agility, as normal, does not influence movement distances). If you succeed and your target succeeds, or if you both fail, then you make no progress. If you fail and your target succeeds, you've been noticed.

If you've been noticed, you may immediately attempt a Concealment test in order to move out of sight before you're spotted properly, or a Decieve test in order to pass yourself off as someone that should be there. Obviously, being stood in a fortified complex, wearing only a black bodyglove and carrying an assortment of dulled mono-knives means that trying to bluff your way out of the situation is going to be very difficult, so modify the difficulty of any Decieve tests according to the circumstances.

If you manage to get within 1m of the target without him noticing you (congratulations if this is the case...), you may attack. Make a melee attack roll at +30 (the target is unaware), modified by other circumstances (such as placed shots, or ambient conditions like rain or snow). This will either be an attack with a weapon (either a melee weapon, or a pistol), in which case you roll for damage normally, ignoring their wounds score (the blow is an immediate critical hit) or a grapple, in which case the enemy suffers a -30 penalty on their Agility test to avoid it, you get a +20 bonus on your Strength tests to continue the grapple each round due to better positioning (this bonus is removed if the enemy wins a grapple test), and the unarmed damage you deal ignores their wounds score (dealing immediate critical hits).

Regardless of the outcome of the attack, your target is aware of you. Grappling prevents a target from crying out, at least until he succeeds on a grapple test against you, but other than that (and possibly the use of a Stummer), incapacitating or killing the target are the only options to avoid drawing attention to yourself. Another stealth test (Concealment or Move Silently only in this instance - Shadowing applies when attempting to follow someone, and that isn't what's being attempted here) is required to successfully leave the scene of the crime without being noticed. If there is obviously no-one around to observe you, you can spend time hiding the body and leaving/hiding at your own pace.

Again, this isn't an automatic win - approaching the target is difficult, and the weapons that're easiest to conceal for these purposes tend not to deal that much damage (knives, unarmed attacks) or are noisy (pistols). In the former case, talents like Crippling Strike (+1d5-1 extra critical damage with melee attacks), Precise Blow, Quick Draw (so you don't have to carry your weapon openly for too long), and Street Fighting (+2 critical damage with unarmed and knife attacks) are all useful, as are silencers for pistols, synskin, camelioline cloaks, stummers, shock weapons, toxic weapons, etc...

I'd rule the target counts as helpless.

But then I'd just let the assassin insta-kill the guy anyway, unless it was dramatically important that there was risk of failure.

Seems easily accomplished if used with the mook rules from DotDG

I've had my fair share of Assassins at my table.

First you need to decide if the the target is important enough to be killed with a single blow/shot.
Does it add to the game, does it add to the drama, does it add some coolness?

Second, the target must be surprised, your not going to achieve an insta-kill if the target is aware of you, unless you use the "Mook Rules" from CC.
The armour also counts in this one, if the target has Full Power Armour including helmet I find it rather unlikely unless you manage to tag him with a Melta Charge (always a good plan and usually yield great and funny results).

Third, the GM has the final call, if it helps your campaign along, you agree with it or think that guardsman should die by a cool players action, go for it.

One important thing to remember is that rules are there to guide your story and not to hinder it.
The story always comes first and as long as you keep the suspense of disbelieve important for you campaign there is nothing that should stop you.

In most combat I use the mook rules, I treat guardsman, gangers and such as having only one wound.
If the Acolytes want to infiltrate a base/camp etc. and they think of a create plan and more importantly, roleplay I allow them to make instant kills, if the roleplaying is loosy I will punish them for that rulewise.

Just look at the difference:

Bad:
I use my conceal skill to sneak up on the baddy and swing at him with my powersword.

Good:
Callidia dissapears into the shadow, she stays close to the rockrete walls to keep out of sight, as soon as the compound guard turns the corner she covers his mouth with her hand and with the other hand she slides her stilleto into his back.
The guard twitches as she turns the blade going limb immediately after.
Callidia quickly hides him in the corner and continues deeper into the compound.


The first example is simple bad roleplaying and a bit of powergaming, using your most damaging weapon and not even trying to describe your action nicely.
I would use the normal rules and if there is some bad rolling you are sh*t out of luck.

In the second example there is flavour text and an assassination that is by the book, cool and visual.
The guard only has 1 wound and I would give some bonuses to the die rolls.

This way you motivate your players to describe their action which is important because it helps players to visualise and become more deeply involved with the game.

Santiago...