https://wiki.project1999.com/Beholder%27s_Maze
I like killing the 2 Minotaur guards where the temple is. Just don't get too close to the eyes. It's afk camp but the spawn time is pretty low. I've done this to 20 on 2 characters. The weapon they drop will also vendor for a few plat.
Depending on your gear, you could do aviak guards in Lake Rathe at 18. I wouldn't really try it without some twink gear as SK because some will still con as yellow to you.
I believe the Karana zones are full of stuff for you to kill if you're into pulling wandering mobs back to a safe spot.
Also took the boat back to Kunark today on my necro and noticed a lot of the ogre's at the dock junction were blue to me(lvl 20). Not sure if they are on OT faction or what so I'd look into that before going there. Seems to be undead and bandit options in the ocean zones for our level range.