Best place to late game level grind?
Depends on what you want to grind. For blood echoes,
The Nightmare of Mensis seems to grant mad souls when killing those tall creatures that occacionally chuck big rocks at you. This is, ofc, only after you've dropped the eye of Sauron to stop the damage/frenzy. They seem like the best source for hassle free echoes, since they are so easy to kill.
If you want to get a bunch of bloodstones, especially dem chunks
kill the blue eyed werewolves at the upper cathedral ward. They have the chunks as a rare drop. Equipping two 'eye' runes and the best moon rune nets you a decent smattering of echoes and with each kill, a chance at a chunk.
I accidently sent bandage boy to the church but it was cool because then I got that cool fight against himWhen he goes beast mode!
What changes if you send him to the clinic instead?
Pretty much everything. Iosefka turns him into a harmless blue mushroom dude who, unlike some other shrooms, doesn't even fight back. Only shivers in a corner. You still get the same rune for killing him 