It sounds as though you may need to increase your Attack rating. If you are using melee weapons (swords/halfted weapons) your strength will help to increase your attack rating. If you are using ranged weapons, your dexterity will increase your attack. Some of the skills that you select will also help to boost your attack, such as Sword Lore, Halfted Lore and Ranged Lore. There are also numerous items that can help to boost your attack. I believe someone else already suggested this, but if you can, socket magic, poison or ice into your weapon/s, as the Kobold Chieftain is weak versus these types of elemental damage.
Try to keep in mind that all of the bosses, including the mini bosses, have a high defense compared to regular mobs, which decreases your chance to hit them. Having a high attack rating helps you out by allowing your character to hit a boss more consistently.