How can oxistat lotion be used for severe acne?

Worthless: Oxistat is oxiconzaole. It's a fungicide, and a pretty good one, but it won't touch your acne. Further, your question says to me that you've not taking advantage of medications that do work. If your acne is severe, I'm going to assume that topical benzyl peroxide, retinoic acid and/or salicylic acid have failed. Get with your personal physician. If you're offered isotretinoin, be brave & do it.