Wolfspider-
Level 6 Rogue, with a +1 weapon, probably does around 4d6+3 damage on a sneak attack with a shortsword. Average damage is 17.
A warrior has a d8 hit die, and if he's constructed with a standard ability score block, probably a +1 hit die.
If you set him at level, lets say 4, his hit points will be 22. So that's too high. You want the rogue to probably kill in one hit. So lets set him to 3. Now his hit points are 16.5. Hmmm... probably still too high, because between the attack roll and the damage roll the rogue will have less than a 50% chance of killing the guard. Drop him to level 2. Now he's got 11 hit points. This seems reasonable. The rogue will almost definitely get his kill.
But wait! At this level, a wizard does 3d4+3 with magic missile, for an average of 10.5. The fighter will do 2d6+7 or so with a greatsword, averaging 13. Only the cleric is really left out of the game.
So hit points at this level mostly determine whether it will take 1 or 2 hits from these pcs to kill the guard. The idea that the guard will require 3 hits to die is almost unheard of.
So... you've created a mook. Why not just use mook rules and save yourself some time, particularly as mook rules let you give the guard a better attack bonus so that, if the rogue doesn't succeed at his sneak attack, he can have an honest (if brief) fight?