I want to have a bunch of kobolds (atleast 300) to attack this small town that was stolen from them by the current inhabitants. The defenders of the town would be a 2nd level rogue with a +2 composite longbow. A 2nd level rogue paldin with a longbow. Two second level fighters & finally a expert on explosives that throws grenade like objects. The town is surrounded by a 12 foot stone wall on three-sides and a mountain behind it. The kobold hoard is attacking mainly from the side with the drawbridge. The cities defenses include a moat, and an array of mines & pits surrounding the fields to inhibit the kobolds (the mines do 6d6 as well as the grenades). Also there are large torches placed around the wall to inhibit the kobolds night-vision. Half the hoard are armed with crossbows & slings while the other half have spears. There is also a small siege party to try to get through the walls/drawbridge. There are other people in this town but they payed the adventurers to save them and will only fight if the kobolds make it in. The townsmen are mainly commoners using swords and slings. So the main question is how many kobolds should i use to make this battle a challenge but not kill everyone outright.