If I think that the japanese economy is going to crumble. Debt to GDP @ 190%... which instruments would be the best to purchase.
At some point their bonds are going to crash, but timing it is a problem. And then they might even default.
Short JGBs... You can even do it in futures. Be aware though that it's been tried a countless number of times by a lot of very smart people. Ended in tears almost every time.