The only way to learn Japanese quickly is to go to Japan. Sign up for the most intensive Japanese course you can find, stay with a Japanese host family, and after class, hang out exclusively with Japanese people. If your head doesn’t explode, you’ll learn Japanese quickly. This is the sink-or-swim method. It can be painful, but it’s definitely effective.
You can also learn Japanese quickly by moving to Japan and marrying a Japanese person. This is only sort of a joke. People who live with someone who speaks another language (and preferably who doesn’t speak their own language) learn to communicate very quickly. Daily exposure, a patient audience, and lots of practice are all part of it too. Japanese is a difficult language. If you meet a fluent Japanese speaker who isn’t from Japan, it’s quite common to discover that they’re married to a Japanese person.
So in short, if you want to learn Japanese quickly, a full immersion experience is the answer. And since Japan is the only place where people speak Japanese, that’s where you need to go. ボン・ヴォヤージュ！