Forest of the Dead. Corrupted Guard, Corrupted Knight, Trampled Man etc. are undead there and drop herbs. Just be prepared to spend some time looking for an available spot..
Solo? Ermnmm... the best way to lvl your bishop between 65 and 70 is to make friends with some pole users (especially destroyers) and level that way....
If you really want to level solo though, your not in much luck. From ~64ish onwards, you can level off doom knights. However, for some odd reason, they dont drop herbs... even tho they are in the open. A bit before 70 you could go to Forest of the Dead and level off the few undead in there, but there are too many aggro non-undead which you will need to trance/peace. There are some undead up the top of BS that drop herbs... however they are outnumbered by aggro orcs. The pass near Swamp of Screams has 1/4 HP undead at night... dunno about herbs. Cant really solo of those much anyways... they're only around 1/4 of the time. Entrance to Imperial Tomb has undead... but no herbs. The passway to get to IT has undead, but also aggro ants/beetles.
So in short, above 65, the only places that are all undead dont drop herbs, and the places that do drop herbs have more than undead (and hence you'll waste mp on peace/trance).
I'd go doom archers -> doom knights -> WoA to IT passageway / FoTD -> Entrance to IT if someone put a gun to my head and told me to level my EE solo to 70. Course otherwise I wouldn't touch solo on a healer with a 10 foot pole.
There are good areas near Disciples/Dev Castle with the doom mobs, away from the aggro trees but like everyone said there are no herbs so unless you have a recharger it will be pretty painful slow.
There are entire fields of mobs that are entirely undead and VERY vulnerable to holy magic.
Avoid the humanoid casters...
Later on in life (67-68) you will have to move to fields that change during the evening to aggro non-undead... there is a field across the road that has all undead during the night to compensate...
Ill give the first one away.
The first spot is behind the mage who give the quest at the port-in spot... there are others actually and this one is often taken... but its an example...