Another solution is that a method of attack that is annoying to player
The aim isn't to annoy, however. It's to kill before getting killed yourself, and, failing that, to escape.
For example try to arrange for "rocks fall everyone dies" while the player/party are walking down a mountain path.
<== Fuck your deus-ex-machina in the ass. Hard, without lube.
Another option would be trying to frame the player for something.
<== full scale quest design.
imagine you walking down the tract. Three mages from hidden places cast Grease, web, and Hold Person on you. 4 archers start peppering you full of arrows from far away. You certainly can take them, but they are surrounding you, meaning you are going to go from one direct to another to kill them one by one, meanwhile still getting arrows on your ass. What the fuck!
The key is maximum range to attack, with status-affected spells, because direct damage spells can be shrugged off.
4 archers 3 casters is already quite a bit above your usual band of brigands.
Or a stealth thieves after your ass. They stealthily sneak to your ass, pick your pocket, then run like hell. You have one or two turn to deal with that fucker or he would have been out of range and you cant catch up.
The key, is the stealth level is changed according to level. At low level, typical thieves with normal invest in stealth. At high level, it's a special race with max invest in stealth. For example 3.5 DnD rule, at low level it's human with max invest in stealth, at high level it's Gnome with nondetection, bonus +2 to Hide outside, max hide, AND equal level to player.
Or how about no, at least to your blatantly level scaled and racially homogenous would be thieves?
<== Fuck your lack of understanding . in IWD/BG setting, Gnome got bonus to Hide, meaning a Gnome will stealth better than most.
First thing first, in order to have NPC thieves game needs a bit of asshole UI - player shouldn't be helpfully informed the moment he loses something from inventory, unless he willingly gives that item up. <== Fuck your blind play too. I like my screen inform me fully that one fucker has just roll a successful steal of my 1 gold coin because I am going to find that thief and fuck them extra hard.
Anyway, thieves, brigands and occasional would be duelist are fine, but I think you guys are missing the obvious:
In general it's often baffling how civilized, in the modern day sense, supposedly rough fantasy regimes tend to be.
Fantasy settings as implemented in cRPGs, even supposedly gritty and grimdark ones, are surprisingly fair, especially towards the player.
Has anyone ever seen local noble or another person in a position of power wanting the PC to hand over some sweet artifact weapon they just obtained and using all means at their disposal to get it, even attempting to murder or imprison the PC if they refuse? And why the fuck not - it's not like most of those settings are democracies, republics or even constitutional monarchies - the guy would be in the right in such case, and having to choose between becoming an outlaw through no fault of your own or giving up something that could really make the difference in your quest would make an interesting choice, especially given how organic it could be in terms of implementation.
In pretty much every cRPG you can pretty much walk into a local noble's palace, insult him, shit on his carpet and just walk out - why?
Or why can't you be wrongfully accused of something as generic mechanics instead of carefully scripted plot-hook?
A king, major noble or another person of influence would probably love to get their hands on an artifact last seen about a millennium ago when it was used to write a good chunk of region's history in addition to a good poertion of local myths and tales.
And, in a feudal or similar system they would have little to stop them or make them reconsider.