I have to say that killing mildew is one of the most dreaded tasks, you will find yourself having to do, but I should say that it's not as hard or difficult as many people make it be. If you spot the issue in its initial stages and have the right tools at hand, than you are more than qualified to deal with the problem yourself.
Cleaners Berwick advise that the first step should be to identify the problem and assess the damage. You have to be sure that the mildew has not grown deep within the trusses of the wall. Once you have done that it's time to start the actual cleaning. What you need is to dilute Clorox in water and use this solution to thoroughly clean the whole area, though I suggest you have a go at the whole wall just to be on the safe side. Scrub using a stiff brush in order to work the bleaching solution as deep as possible and kill off the spores that cause mildew to grow.
You will notice that immediately after the cleaning, your wall will look better, but don't rest on your laurels as the mildew might not have been a minor surface problem. If in a few days the mildew reappears, then you have to take a lot more serious measures in order to get rid of this. This means cutting through the drywall. This on itself is not that hard. You just need to calculate, which is the contaminated area. To be honest, even though the stain visibly ends some place, I have to say that a lot more is hidden.
Cleaning Berwick teams advise that when you consider what part of the wall you should cut make sure that you are cutting a good five inches around the stain just to be on the safe side that no spores remain. When you consider the shape of the cut, you better aim for a more geometric shape, which will easily be replaced later on.