Living in Florida and choosing not to use Air Conditioning, I know about mildew.
Drying it out and keeping it dry is essential. Activated Charcoal will get rid of the odors, you can usually find Activated Charcoal tablets (for human consumption) in an old, independent drug store here in the US.
It also helps to put your case in the direct sunlight. Mildew doesn't like direct sun. Bleach kills mildew, but I'm rather reluctant to use it on my guitars or case because I don't know if it will harm the finish or not - perhaps someone who knows can chime in here.
Paradichlorobenzene moth balls will keep mildew away in a closed area. Back before AC was common in South Florida, people used to put packs of this stuff in their closets to keep the mildew and mold away. Of course, this is a potentially nasty chemical so after use, air it out well. Anything that will kill the mildew, isn't very good for you either (including bleach)
But the Activated Charcoal tablets you find in the drug store are safe. Hospitals administer Activated Charcoal to patients with food poisoning. I always try sunshine and charcoal first, as they are non-toxic. Sunshine kills the mold, and charcoal removes the odor.
I keep my guitars out of the cases, and regularly air out the cases.