I mean there's nothing wrong per se with boiling, but that is going to leave a hell of a mess. All that fat is going to melt and float to the top of the water, not to mention the blood needs to be drained. Neighbors might wonder what smells so good, too.
I also don't get why he says he can't get a strong acid, unless that stuff is heavily restricted in Japan, because you can make it with chemicals that you could get at any hardware type store.
There's a form of cremation that uses a basic water solution actually (alkaline hydrolysis - and not an acid), which is 95% water, 5% potassium hydroxide, then heated to 350 fahrenheit to basically turn a human body into a liquid that can be safely poured down a drain.
The high-pressure, industrial version of that takes just two or three hours. At the end, the bones are still there, but they're very brittle and can be easily ground up into a fine powder, which of course is easily disposed of... you could just spread it in a garden or dump it out in a stream or something.
You can buy potassium hydroxide on amazon or at any hardware store - it's used in things like soapmaking and it's cheap - $15 or less for 2 pounds of it, which is far more than you'd need to get rid of a corpse.
Obviously doing this in a bath tub is going to be pretty difficult, so moving the body is necessary, but if you had a large steel barrel you could super easily light a fire underneath it and slurrify someone in a hurry.