The Mist Wraith on screen back respawns on re-entering the map and it's probably the best (yet still painfully slow) way how to level. You may level faster at the Mermaids, but it's for Random and Sarah only and you need the levels for Cocka-Cocka, too.
From my experience, it's possible to kill 6 Wraiths per minute, it is around 700 per hour so something like 30 000 XP and 2000 gold per hour.