Here we can see Sukuna running around killing monsters with his deadly slicing attacks.
Sukuna, as a player of the game, knows the mechanics of dungeons, and hence has formulated a technique to gain Cursed Energy Proficiency in the most efficient way.
Monsters in dungeons increase in strength as one goes closer to the boss room, and these monsters give more EXP.
He has figured out that the EXP required to level up increases exponentially with each level, and each level up gives 1% Cursed Energy Proficiency.
Monsters use the mana from the dungeon core to be spawned, once a room is cleared out, the monsters start respawning, decreasing the amount of mana stored in the core.
Sukuna has chosen two rooms, the closest he could find to the boss room, to repeatedly clear.
He kills all the monsters in one room, then moves to the other room, killing all the monsters in that room. If, somehow, he clears the room too fast and there are not enough monsters respawned in the other room, he uses a method to increase his Reversed Cursed Technique Proficiency.
At the start of the dungeon run, he picked up a dog-like E-Rank monster. If he has to wait anywhere, he lets this dog nibble on him, by nibble he means eat his limbs off, then he heals himself, which increases proficiency.
It is really surprising that a man has such a strong will that he does not get fed up with just stabbing monsters all day long, and being eaten a few times.
He has been doing this for two hours, and the only thing that keeps him going is his chant.
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'GIMME XP XP XP XP GOTTA GET THE DOMAIN'
'GOTTA GET THE DOMAIN'
'GOTTA GET THE DOMAIN'
'GOTTA GET THE-'
[Level up! Cursed Energy Proficiency +1%]
[Cursed Energy Proficiency has reached 60%]
[Unlocking Fire]
[Done, Chant 'Open' to unseal when needed]
'LET'S GOOOO'
He finally reached what he was aiming for, he decided to take a little rest before continuing.
What? You thought he would just stop after getting his milestone? Nah, he isn't going to leave such a good EXP farm without milking it down to the last drop. Who knows how much time before he gets to enter a B-Ranked dungeon again, one with so many mobs even.
He sat down in the corner of a third room as he pulls out the dog from his coat he uses as a bag.
He sighs and looks at the ceiling, he had been having an existential crisis every time he lets this dog eat him alive. Killing monsters repeatedly without break made him so bored that he just wanted to destroy this dungeon to atoms.
'It hasn't even been that long and I feel like this, I'm so sorry, Geto, for thinking you had it easy'
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A few minutes later, we can see Sukuna running around the two rooms again, while slashing the monsters with his knife-like weapon.
The knife rusted due to deprivation of Cursed Energy in the environment, and now with Sukuna giving it plenty, it became shiny, like really shiny. It's edge was so sharp that it could be pushed into a stone wall by a rat.
Sukuna started using this weapon about an hour and half into his grind. Why, you ask?
Well, this is because . . .
On the day Sukuna got the knife, he practiced with it on the field for a while.
He went to his dorm room and tried to keep it on the table to find a good place for it, but it just suddenly disappeared, while he got the notification
[Inventory Spaces Occupied 1/4]
Well, that's great, he didn't know there was an inventory in his system, but there was a problem, he didn't know how to get it out of there.
He tried a lot of things.
'Inventory'
'Inventory Open'
'Storage'
'Item Storage'
'Item Reservoir'
. . .
It didn't work.
But suddenly, as he was grinding, the knife popped up in his hand.
He didn't know how he did it, or how to do it after this, even, but right now, this was very beneficial, his efficiency increased by a huge margin, but the difficulty of increasing Cursed Energy Proficiency also kept increasing.
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After another hour of doing this, he was still quite far from unlocking Domain Amplification.
This is when he sensed creatures coming towards him, and he recognized them.
"Guess who finally decided to come" said Sukuna as he walked towards them.
The four had uncertain expressions on their faces, looking at the bloodied clothes of the grinning guy, but as they walked further and looked around, those expressions turned into ones of shock.
"S-Sukuna, what were you doing here?", said Anny. She seemed the most shocked out of the four as she probably understood the mechanics and power levels of the monsters best, and she might have already figured out what he had been doing.
"Farming mobs of course, isn't that how people level up?", said Sukuna nonchalantly as he walked to his resting spot.
'This is not a game dammit' thought Anny as she was speechless. They had problems killing a single C-Rank, but this guy had been repeatedly killing them like killing mosquitoes.
The other three also figured out what he had been doing and stood with their mouths agape.
Ryogaku and Alexia had admiration in their eyes, while Johan had determination, to surpass him, probably.
"Well you all have grown stronger I see, Rank tw-no, Ryogaku, especially" said Sukuna as he got up and walked towards the huge decorated door of the boss room.
"It seems you all had plenty of rest before coming here, so get ready for the final battle, we'll be going right now, if there are any objections voice them now" said Sukuna with a slightly serious voice.
No one said anything for a few seconds.
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"Alright then, we're going in now" said Sukuna as he pushed open the door to the boss room.
The door slowly opens, revealing . . .