I agree with not using slugs. It does reduce the chance of a hit. I would go for either #1 birdhsot or a 34 buck.
One idea- practice rapid reloading. With one shot you need to be able to relaod extremly quickly. For example, when you drop a load of sheels in your pocket, shake the pocket. so the weight of the lead puts the primer end upwards. If you can, try holding spare rounds between your fingers, like cigars, and try that as a way of reloading quickly. Practice firing from your off shoulder, and in poor lighting. All that stuff helps, but most importantly give syou confidence in yourself and the weapon. A .410 is not ideal for slef defence, but will kill a man and is a lot better than throwing rocks.