Can you unlock because it's there in boss rush?

[Paiguy]

I've been doing a nightmare playthrough and I can not for the life of me make the jump to get to maju's room. I've been trying on and off for days now and I just can't make the jump, be it a normal or dash one. When I wait too long to get close to the edge for max distance there is some sort of input lag and I end up just falling off and doing a downthrust, but if I go too early I just can't get far enough.

Back when I did boss rush after my first playthrough majunun appeared at the end of it. Can I get credit for beating him in there or will I just be out of luck since I can't clear the jump boss?

[Paiguy]

So I did finally manage to get in, but I'm just doing 1 damage even at level 52 with max level swords. Went back to my hard mode file and got the same. I really don't feel like doing a full playthrough on normal for the sole purpose of this one achievement so I guess I'm just stuck missing this last one unless I get really bored.

PS you can't get it in boss rush I tried.

[Iced_Paper]

Are you playing in Catastrophe mode? If so you can try and grind gold to get power (or defense) seeds.
You're supposed to face Majunun at level 54/56 in Nightmare iirc, but that's A LOT of grinding just for that. I want to say "do it on Normal-Catastrophe, it's easier and faster" but do as you wish.