People commented, that its faster to just do NG+

So just do that, if you dont care about resetting.

1. Travel to Subterranean Lab B4
   
2. Equip both XP Relics (Ribbon and Blade)
   
3. Equip High Damage and Mobility Spirits (Level 5s and Incompetent Sinner)
   
4. Move left and wreck the block, then the two flying things
   
5. Move down with the Downslash and kill the two Uppercut guys and the last two flying things
   
6. Escape and "Return to Respite"
   
7. Repeat

Go down to Subterranean Lab B4. This is an Endgame Section of the Game and i recommend you upgrade your spirits to level Max. This will make the grind much faster and easier.

To prepare i recommend equipping both Exp Relics. These are the "Bloodstained Ribbon" and the "Blightwreathed Blade". Check out the map guide to find those, if you havent already.
Also use all three damage relics (Blighted Appendage, Ancient Dragon Claw and Giants Ring) for more Damage and even faster farming. Ignore most Health and Defense Relics because dying does not matter. You simply respawn and lose a little time.

Next up for spirits i use the Imbral Knight (Level Max) and the Incompetent Sinner (also Max).
Use the Umbral Knight as your main damage and the Sinner for Mobility. If you havent tried the Sinner yet I recomend you do, as this spirits is very useful.
Your other spirits dont matter, just pick what you like.

Now the route:

1. Go left with the Spectral Lance dash attack (its faster).
   
2. Hit the Blob thing with the lance and hit him rougly three times with the Knight.
   
3. Usually he attacks now, so dash through him with Sinner (This ignores his attack).
   
4. Now attack until he is dead.
   
5. Afterwards kill the two flying things with the knight or any other spirit.
   
6. Next up: The Uppercut Guys down below.
   
7. When on the Platform: Drop and immediatly use the Giants Hammer (down slash) attack.
   
8. This stuns the guys and you can take them out.
   
9. Now go deal with the last two flying guys (if they are aligned right you can Sinner dash through both and kill them simultaneously).
   
10. Press Excape or Start and press "Return to Respite". No need to walk back.
    
11. Repeat the whole thing.

These number are very rough and are calculated with both Relics equipped.

The "Blight Blob" : ~2000 Exp.
The "Flying Monster": ~500 Exp.
The "Uppercut Guys": ~1000 Exp.

By adding up one Blob, four flying Monsters and two Uppercut Guys you get about 6000 Exp.
I did one run every 30 seconds.

If you start at level 90 and want to go up to level 100 you need about 355.000 Exp.

This means you will spend about 29 Minutes fighting these guys.
Its a little tedious but its definitely doable.