Several categories of guards (I'll simplify to 3 groups):
- rookies, you can solo couple of them in same time
- average guards, balanced for 1 vs 1 with a player
- pro guards, couple of players are needed to bring only 1 such guard down
Those guards would be placed appropriately to importance of place they are guarding (are they only guards on entrance to the city, road patrols, or are they guarding governors halls, etc.).
Normal NPCs (vendors and such), non combat trained NPCs would generally be an easy kill, but if they are in city, there would be guards nearby to help them if they call for help. Perhaps some rare NPCs (weapon smith, class trainer, where it makes sense) would also have some skills, and could combat the player efficiently on their own.
Young NPCs should run faster then old NPCs when attacked.