Scientific method to microwave octopus
If you want to boil something in your microwave, you certainly can. I doubt it'll be as power-efficient as doing it on the stove, but it's possible.

But there's no formula to get you a time and power level. You probably want to heat on maximum power until the water boils, then heat on the lowest power setting which keeps the water boiling until the octopus is done how you want.

On top of that, depending on your microwave, even the lowest power setting may be too high, and cause water to boil away enough that you'll need to replenish it eventually if you're cooking for an extended period.