There won't be a lot of good answers to your question. The game assumes frontal assaults wherein sides engage each other in the open. No iteration of Dungeons and Dragons has been a particularly good stealth simulator because such a game needs to be built that way from the beginning, and Dungeons and Dragons and its ilk often betray their wargame roots by making what you describe extremely difficult.
This is the go-to for folks without spells. Yes, 1 in 20 of them poison themselves--and that's hilarious--but the rest successfully smear drow poison on their weapons (at home! so if they poison themselves it's private!) and stab folks into unconsciousness, smear blue whinnis on their weapons and stab folks then run away and wait to see if it works, or drop some oil of taggit (or, I guess, oil of restfulness--whatever, Pathfinder SRD) into the guards' coffee pot and watch them all nod off.
The problem with poison is acquisition (it's usually illegal) and cost (it's expensive to equip), but it does exactly what you want while not needing the character to commit a much more valuable feat resource to the task.
Pathfinder's rules for settlements exclude the Dungeon Master's Guide's precise ability to determine every ctizen's class and level, leaving such tasks to the DM, which is fine, but it means the DM has no guidelines as to what classes and levels most citizens should be, which is also fine as that should vary by campaign.
So, if the goal is to have guards easily knocked out, make low-level guards the norm and let the PCs down them with nonlethal damage. This has several advantages.
- Encounters are quickly generated.
- Encounters are quickly resolved.
- Encounters of this type make the players feel like their characters are badasses (or bullies or monsters depending on the players).
- Encounters are worth a tiny amount of XP, if any.
- Encounters are monetarily unrewarding.
So if most folks standing guard in a random spot are 1st-level dudes with, like, 6 hp, PCs of just about any value will be able to use their weapons to inflict sufficient nonlethal damage to down them with single attacks, but doing so is just a method to getting to the actual encounter with something that matters and will provide an actual challenge, and failure in guard elimination means activating an encounter early, which could be bad, or, worse, activating several encounters early simultaneously, which could be really bad.
This requires a conscious decision on the DM's part. It means most of the city guard will not be threats to PCs after about level 6--it'll take 19 guards committing suicide to get 1 who manages to hit a PC--and the PCs will quickly only worry about other dudes who have serious class levels. That's okay if your group isn't a bunch of spree killers and understands the world created (I mean, really John McClane doesn't commit bank robberies but stops them, and Jack Bauer's only robbed one convenience store and that's because he was running out of time!), but the campaign will be difficult to manage if the group is anxious to explore their baser instincts.
If you want to encourage ambush tactics over frontal assaults, ambush tactics must be rewarded. The easiest way to make this happen is a house rule: "A flat-footed foe unaware of an attacker's presence is helpless versus that attacker, and a failed Fortitude saving throw versus a coup de grace administered by a weapon that inflicts nonlethal damage causes unconsciousness for 1 hour instead of death." Instantly, the whole game changes: PCs who sneak up on foes can take them out even though the foes are awake, and this tactic makes sneaking around to quickly end encounters an actual thing without needing to drop folks into pits filled with contact poison or mandate all guards be wimps.