Hundreds of authors out there, yet one choose to compare to Oda, why, Horikoshi isnt and cant be Oda. HeroAca isnt and cant be One Piece. I dont think or at least hope people dont want One Piece clones.
I'm not saying, that the manga should copy One Piece. There are just tools out there, which help to create a story with a fast pace, many charakters and in a tight space, while still making it focused and fun to follow.
Another similar example would be the new Judge Dredd Movie. It plays only in one apartment complex, where almost anything looks the same and is overwhelming grey. But the film producers used tools smartly to give the audience a better chance in following the story.
First the heros start at the first floor and the bad guys controll the top floor. Just with this information, we know the goal of the story: get to the top. The apartment complex is designed with a big open space in the middle. This is a great orientation point and the audience can through the movie get informed, how far the heros are away from reaching there goal. Second the bad guys constantly inform us, on which floor the heros are at the moment "They reached the 30 floor!". Third all major players are introduces before the party starts, so we know about the challenges on both sides. New enemies join the action at the end of the movie, but they are greatly introduced, fit the theme of the story and the audience know more about them in comperation to the heros, which makes for a exciting encounter (classic bomb under the table scenario).
I think, the bad guys in the basement arc should simply have a map, where they coordinate there plan and update informations (instead of the cliche boardgame). This would inform the audience about the progress both sides are making and build up exciting encounters, since we know about there opponents but not the heros.
Also it is a basement, there should be different rooms, which could create exciting encounters and backgrounds. A boiler room, storages, maybe a barrack, a labor, etc. Not just hallways (as a Pen n' Paper Dungeon Master this is one weak dungeon).