The commoners (assuming they're human)
- Have d4 hd, and no con mod. If they're hit, they're dead.
- Will have no armor.
- Is proficient in one simple weapon, likely club (it's commonly available), staff (ditto), sling (for hunting), or light crossbow (likewise). Most of them won't even have ready access to the one weapon they know how to use, and will be using tools as improvised weapons... taking a -4 to hit.
- Have Listen and Spot as class skills... but no general motivation to put ranks into the skills. With their 3 skill points per level your average commoner is going to be putting their efforts toward: A Craft or Profession that earns them their living, Handle Animal (which, let's face it, is pretty essential for most commoner-ing), and a few points here and there to Ride, Swim, and Use Rope. Unless you want to stat out every single commoner to account for the exceptions rather than the rule, you're looking at a general lack of awareness.
- Cannot see in the dark.
- Will likely spend their feats in a fashion similar to how they spend their skill points. Skill Focus (Craft or Profession) should be at the top of their list of feats to pick. Beyond that, Endurance, Run, Animal Affinity, and possibly another Skill Focus would be appropriate.
- Some of the commoners will be non-combatants. Elderly, young, scared silly, etc. All of the orcs showing up will be battle ready.
The bog-standard MM orcs:
- Have a level in warrior; +1 ba, d8 hd, proficiency with martial weapons, armor, and shields. Add to that the fact that the orcs have both the means and motivation to show up packing. They're the aggressors after all.
- Have a +3 str mod and a +1 con mod. They're walking in at a +4 to hit the commoners AC 10 (connect on a 6+). The commoners, with their +0 ba, no str mod, and good chance of having to eat a -4 penalty to hit for using an improvised weapon are going to have a much harder time tagging the orcs' AC 13 (connecting on a 17+). Unlike the commoners, the orcs can take a (uno) average hit and keep going.
- Have 60' darkvision and are packing ranged weapons that can likewise benefit from their str mod. Likewise, because they can see in the dark makes sneaking up on the commoners rather simple. Approach under cover of darkness, taking 10 on move silently and hide. Assume anyone on watch is also taking 10 on their appropriate notice-what's-up skill. The orcs have a -1 ACP from their studded leather. Counting in the range penalty (-1 per 10'), that means the orcs can get within 30' before they're spotted.
Well within javelin range.
Just off the top of my head, an appropriate tactic for the orcs to use would be: sneak up under cover of darkness, kill anyone you come across, then light a couple outlying buildings on fire. Move off a bit, then when the townsfolk come a runnin' to deal with the burning buildings, hit 'em from the side and slaughter everyone who showed up. Then move into town killing indescriminatly as you go, maybe occasionally lighting another building on fire to keep people confused and panicing. Drag off what you want, and leave the place a charred sepulchur behind you when you go.