Bloodstained: Ritual of the Night

The clip should be self explanitory but incase i embedded it wrong or it disappears I'll explain.
Ok, the simple way to do this is as follows.
Step 1. Go to the 8-bit dungeon located in the hidden room above the warp room in the Hall of Termination.
Step 2. Bring along a powerful Greatsword or something that swings above you.
Step 3. Continue through the area until the 8-bit zombies start flooding endlessly, Invert so that you are on the ceiling the place yourself on the "stairs" directly below the flow of zombies.
Step 4. press the button over and over.

I went from level 35 to 54 while I made a phone call.
Hope this helps.

An Alternative to what has been Suggested [At least this is what I ended up doing for the 9 or so levels I was short for this]

There is a room, across from a Save point in the Den of Behemoths where a Sole Abyssal Guardian Spawns. though he is worth notably less than Master Carpenter [1781 xp vs his 5024] It also has considerably less health than he does [only 1871 vs 7k]. the Guardian is also right next to the room's door and doesn't call for assistance.

I found it faster to simply Spam kills against the Dragon for simple levels. at around lvl 40 it only takes a good 6ish minutes or so per level depending on your spell and weapon setup.

The grinding method I used was simple. In the Oriental Sorcery Lab, you'll find a boss called the Master Carpenter. I don't know why but he always respawns and is worth about 5000 EXP. He summons up to three Demon Lords which are each worth over 1000+ EXP. He is simple enough to fight once you have the Invert ability and good weapon. Enter his room and attack him from the back, jumping over him as he turns around to hit you. when he starts to build a oujia board, spam the attack button until he walks out of range, then invert and quickly defeat the Demon Lord he summoned. Rinse and repeat for awhile and you'll hit lvl.50 no problem. I'd like to mention I did this after having defeated and which gives you a wild power spike.

Side Tip: If you need to heal, you can just backtrack to the teleporter in the area, go the Hall of Termination teleporter, and heal at the save room nearby, then go back.

Side Tip 2: I used the Weight Ring and Tactician's Book to maximize the EXP.