i like big booba

Best post dragon grind is the Outskirts, but killing every enemy from the original Ogre spawn to the bridge to the castle. Nets like 10k xp in like 1:30 minutes.

Is the Fulminated Mercury farm spot good as well for xp? It feels like it. The Ashina Outskirts-Stairway idol, 2 red guard gunners, 1 flame throw red guard, and 1 red guard longswords?

I think I found a better post-dragon grind:

Upper Tower - Antechamber idol, run up the stairs, circle around to the right of the purple fella, hit him with that backstab, and run it back and rest. Much easier after the first time when all the wall panels and stuff on the floor is broken and don't make noise. Takes 14 seconds (yes, I timed it) to get a kill and reset. Pop a Gachiin if you're not too confident in your stealth.

A much better midgame farm than those listed once you've gotten to the Gun Fort idol is to run backwards from the idol, backstab the scattergun enemy at the exit to the room, then run back and rest. She drops 297 exp and 52 sen (as well as a good amount of spirit emblems and possible other drops) and you can do ~7 runs per 2 minutes, working out to about 1040 exp/min and 182 sen/min.

Fighting the other scattergunner isn't worth it unless you can beat her faster than it would take to run to the idol and back. I've tried using Bloodsmoke Ninjutsu to get a backstab on her, but she's not quite close enough for it to consistently let you get the deathblow.

So, the best farming spot I've found is Outskirts Wall - Stairway idol, during the second attack. You grapple up to the tower, dropping kill the guy in there, drop down on the right side of the tower, back stab the guy there, run the the gate, back stab that guy, then there are two patrolling guys, back stab one, sometimes you'll be able to back stab both. If ya feel ya won't be able to back stab both, go for the guy in with the two swords first, the guy with the big cannon goes down pretty easily. Nets around 5562 xp each run in NG+1, and only takes about 35 seconds each run