Yeah, Kobold Skirmishers are
deadly when used correctly. They do 1d8 damage with a +6 to attack. Mob Attack gives them a +1 bonus to hit per Kobold ally adjacent to the target, and then they get an extra 1d6 damage if they have Combat Advantage.
In KotS I was playing an Axe and Shield Fighter with Scale armor, so that's a 19 AC. During combat, I had 4 Kobolds adjacent to me, two of whom had me flanked (I hadn't read the MM at the time, so I didn't know how deadly this was). So, let's take a look at how that plays out...
The two Kobolds without CA have a +3 to hit, because of 3 allies adjacent to me, making for a +9. On my AC of 19 that means they hit on a 10. That's pretty good, and means they hit a little better than 50% of the time.
The two Kobolds with CA both have a +5 to hit, because CA adds a +2 on top of the +3 from mob attack. So that's a +11 to hit, meaning they hit on an 8. That's
really good. They also do 1d8 (avg 4.5) + 1d6 (avg 3.5) damage when they hit.
Now, IIRC, the two with CA hit and only one of the other two hit. So that's 3d8 + 2d6 worth of damage, which on average is about 20.5 damage. And then my party wonders why I got bloodied after
one round of attacks.