Voidgloom 1tap question

[Cubing-wolf]

So I bought 500m fragged midas and try to 1 tap. I have like 5.3k mana with midas(with painting power stone). When I hit it at 140m hp left(ofc I use end stone sword.), sometimes it dies but usually 1 tap fails. I don't know the difference between those two cases and I want to know what I did wrong. Some guy said I have to swap fd but it is kinda difficult for me(any tip?).

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