Okay, I just tested your level and I admit that it sure was a great and well designed level.
The stage was overall pretty hard and should be put into either the second to last or final world as it requires navigating through a more or less small corridor with limited space so you need to carefully plan ahead and react accordingly (especially when it comes to manipulating Boos; at one place in the second room I misused them as platforms for spin jumping and reaching the next platform that rose out of the lava).
The second half of this stage is considerably harder than the first half yet never feels inadequate hard.
The visuals are very nice. I noticed that this game tends to use backgrounds from the original source and combines those with custom forground tiles (Desert Dungeon, the factory level etc).
As for criticism: Maybe you can give the Fireballs that come out of the lava the same dark red palette as the lava. Currently it looks somewhat weird that said fireballs have the original lighter red whereas the lava is drawn in a darker red.
The blue Koopas that are encountered in the second room could be switched with red Koopas since it fits the tone of the level which seems violet and dark red.
The two giant plants that will be faced in the second half of your stage are way out of place in my oppinion and could be replaced with fire spitting Piranha Plants.
Overall a very well designed late game level you created there though!