When it comes to getting a visa - you might want to check out this thread;
http://www.japanforum.com/forum/livi...ive-japan.html
The only way you can pull it off without a degree is by having some really special skill, and long term experience in that field... For example, I have heard of someone who had extensive (20+ years) experience as a mechanic on very specialized industrial machines getting a visa without a degree.
You don`t exactly need special skills to work in a combini.
You can make a lot and save a lot working anywhere, if you work enough. The combini night shift is on the higher end of the scale when it comes to hourly-wage type jobs... But it`s still only 1000 or so. So... To get that 280,000/month, you`d need to be working 280 hours a month. Working 8 hour shifts every day, with a double shift once a week might get you there.
Of course, that`s before taxes and health insurance. 280,000/month usually gets something like 80,000~100,000 taken out for that. (Differs depending on where you live.)
Why on earth would you want to slave away with no days off just to get by?