Fiction is full of moments when instead of facing an enemy head-on in an open Conflict, an attacker prefers a sneaky approach that, if fully successful, bypasses combat. Whether it's Léon sneaking from the floor below and suddenly pulling the ambushed gangster down the stairwell to his death, or Agent 47 sedating the target's Very Combat Capable bodyguard, or a dangerous assassin getting taken out by a bomb under the bonnet, or a sniper aiming for a one shot one kill, the unifying characteristic of all these situations is that the attacker goes through some additional preparation in order to bypass the conflict (and often also remain undetected in the process, thus not alerting other opponents).
But while Conflicts seem like a situation for which the game-mechanical support is pretty solid and transparent, assassinations and sentry removals seem to mostly fall either under invoking an Ambush Aspect of some sort (which is likely to only slightly tip the scales into the assassin's favour against comparable foes), or some sort of homebrew mechanics (or at least a very distant extension of RAW mechanics that borders on homebrew). Fair enough, it's not the first case when the best practices of doing something with the system fall outside the boundaries of canonical text, and within the area of veterans' experience.
I must admit that this question is partially inspired by succeeding in a one-shot takeout in a session yesterday, but it is exactly the conditions and unlikeliness of said success that prompted me to take a closer look at improving the handling of such situations. (In my case, the opposition was weaker than my character, the dice were strongly in my favour a few times, and the whole thing involved three free Invocations from preparation that involved expending limited in-setting resources and three FP.)
Thus, I turn to the community with the question of how to fairly and consistently handle such situations.
Definitions Used in the Context of the Question
By 'fair', I mean primarily as compared to the default alternative of just getting into a conflict. I don't want the covert approach to unambiguously make the overt approach pointless by comparison. I wouldn't want the opposite unfairness where the covert operators can never take out a target quietly and are better off being replaced by open-conflict specialists because a modest initial advantage followed by a global alert is the only likely outcome. That of course means that there should be both a real challenge and/or risk and a real payoff to the stealthy approach.
By 'consistent', I mean the 'Any tool or technique in the protagonist's arsenal may also be used by an antagonist'. If the hunter can take out the jaguar by a well-placed dart with arrow frog venom, then the risk of being jumped by the jaguar from the ambush in the branches (assuming the hunter and the jaguar are of a comparable tier of threat on their own - comparable skills, numbers of stress and consequence boxes etc.).
'Silent' and 'Assassination'
While not all such eliminations (lethal and nonlethal) need this, a common trait of such actions is that they're meant not to alert anybody upon successful completion. Of course, setting up an assassination or knockout in a way that doesn't limit itself to remaining covert after a success should often be easier than doing so with a maintenance of complete stealth.
A Solution Considered But Not Yet Tried
I've been told of Consequential Contests (where at the shifts of success can translate into Stress done). I have the impression it is somewhat limited in terms of what narratives it will fit, so I'm looking for other, better ways of handling such situations. If I find none, I'll probably put this one in the queue for testing in my campaign, or suggest some variant of it as a player.