Is なんとかなる　only for positive things? this phrase is usually used to describe either getting through a hard situation or overcoming something considerably difficult or unpleasant; ie how will you manage to navigate without your smartphone? Basically i understand it to mean 'somehow or other' Is it unnatural to use it for negative uses of 'somehow or other'? for example - hes really good at speaking japanese in every day life and he studies all the time, but somehow he failed. ... for that situation could you say ? Sounds wierd to be, so if im right that it is weird, what would you use instead there?

No, is weird. It sounds like "I managed to fail", as if you actually wanted to fail. means "I could not manage it", "I couldn't make it, after all", etc.

"Somehow he failed" is {///}.