The walkthrough is also online (spoiler alert!!)
Viewing post in Isa Brave MSX Underworld Keys Extended comments
You leave me baffled; a lot of people say my game is difficult, but for you, it's easy... You're a true master!
The only challenge left for you would be to do it on a faster MSX.
• If you are using WebMSX, select MSX turbo R Europe PAL.
• If you are using blueMSX, select MSX turboR at 87% CPU speed.
Best regards and all my admiration!