>まず、不死身の女の子を用意する。
First, we need an immortal girl.

>穴に入れて溶鉄を流しこむ。
Then, we put her in this hole and pour molten iron into it.

>とても熱そうだ
Looks very hot

>固まる。
It's becoming solid

>なんか聞こえる
We are hearing something.

>３日ぐらい窒息させてみよう。
Let's suffocate her for about three days.

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>割る
Breaking it up

>ヒュー
hyuu (I think this is hardly breathing noises from her, who is still alive due to immortality)

>と、めでるのだ。
And, it's admirable.