One of the places I've always felt most RPGs fall short in selling a power fantasy is with the high level assassin PC. Lots of games enable players to play assassins. Most class games have Assassin - or something like it - as a class, and nothing stops a player from taking money for killing people. However, when it comes to the mechanics, the ability to kill someone swiftly and efficiently from a place of hiding - or while they sleep - is something most systems aren't inclined to...at least, unless the NPC would just as easily be killed in straight combat.
In D&D, for example, a 10th level assassin striking from surprise will do - at max - 80 damage barring a magical item. (5D6 sneak attack + 1d8 Rapier damage, both doubled from the auto-crit for 10d6 + 2d8. Maxes out to 76 damage +4 for an 18 dexterity for 80 total.) An average fighter with a 16 Constitution at 10th level has 89.5 hit points. So an assassin with literally everything going there way for their power fantasy can not take out an equal level fighter with only average hit points (and I even assumed 5.5 HP for average instead of the 6 the book suggests.)
But PCs aren't supposed to fight PCs, right? Right. Except, per the DMG's rules for creating monsters a Challenge Rating 2 creature (a moderate fight for 4 level 2 PCs, Per Xanathar's Guide to Everything, a Level 10 PC should be worth two CR2 creatures for a moderate fight. So 1 Level 10 PC vs. a CR 10 creature should be a slaughter, but even with rolling absolutely max damage in a good setup the assassin can't assassinate the CR2 creature on average.
This is not an uncommon problem in other systems as well. The Edge of the Empire system from Fantasy Flight Games has ways to make an assassin more likely to take out minions and rivals, but one shotting a nemesis is still hard without buckets and buckets of XP. Even grittier systems tend to have rules against getting one shot, and for good reason (because it sucks getting one shot from ambush.)
So, assuming you need a solution, what is the solution to this? As always, there is a key problem you need to solve - or at least be aware of - before you put any solution in place. So what is the key problem?
Good For The Goose, Good For The Gander
The big problem is that whatever rules you have to make assassin's more able to assassinate can be used either way. This means the thing you do to let your Assassin PC feel more powerful and competent, can also be used to make an NPC assassin feel stronger against the PCs. Like I said above, getting one shot sucks in most situations, and the majority of ones it doesn't suck in takes very special and specific players.
Keep this in mind. Much like changing how criticals work in a game, this is a double edged sword. Maybe your players don't want their to be a way to one shot them in the game - even if it means they can one shot monsters. Maybe you don't want that in your game.
Vulnerable/Immune to Stealth Attacks
Shadow of War and Shadow of Mordor have an assassination mechanic in game, but it gets a twist when you come upon the Uruk captains. Some captains are immune to stealth attacks - if you try to stealth kill them it auto fails and you end up in a fight with the captain. Some captains are vulnerable to stealth attacks - they just die when stealth attacked. Others are in between, they take a lot of damage from stealth attacks but are otherwise fine.
This can easily be ported into your game. PCs are always the middle group - sneak attacks hurt, but shouldn't one shot. While some NPCs are able to be taken by stealth with ease, and others are always on guard.
Speaking of on guard..
Ready/Unready For A Fight
One of the ones I've been prone to in situations is based on whether or not a character is ready for a fight. In most systems, your Hit Points are as much your ability to turn a lethal blow into something non-lethal or less dangerous as they are a way of tracking your health. Even with wound penalties, numerical wounds just work better as tracking how able you are to continue fighting more than how many sword stabs you can take before you die (because it's really weird that one human can be stabbed in the heart 8 times and be fine, while others can only be stabbed once.)
Extending this is the idea of whether or not you're ready for a fight. This is different than surprise/not-surprised, but actually how off guard you are. Someone in their home, weapons on the rack and out of their armor relaxing with the family is a lot more off guard than someone walking down mainstreet in their full kit and gear. A warrior with weapon drawn and in stance, is a lot more ready than one with weapon sheathed and thinking everything around them is safe.
The idea here is if someone isn't ready for a fight - if they're off guard and unaware of a threat around them - they can be taken out easier. Being 'on guard' doesn't mean having your weapon drawn, but it does mean being alert and attentive. It also means you can't really be friendly - at least, not unless you're hiding that you're on guard.
Characters with Wary or Paranoid would likely always be on guard. PCs could be on guard, or not, as they said. Attacking someone who is not on guard is easier, and thus able to do more damage. Perhaps even to instant kill. Of course, in a high level politics game that means you can oneshot a PC which may be dangerous.
The more I think about it, the more i like the idea of On Guard vs. Not On Guard. I may have to test it out in a game. But what are your thoughts on this problem? How would you solve it? Do you even care?