I would suggest the following :
17-19 : Parthanan Farm (horse to East Lough Derg)
20-24 : Spraggon Den (halfway between Howth and the Silvermine Mountains)
25+ : Darkness Falls
If we haven't got DF, I suggest either the Koalinth Caverns for 24-30, then Treibh Caillite until 35. After that Coruscating Mine will take you from 35-50 quite easily.