ymmv, but I found the lower-level undead in Arkham to still be very rankable. Especially if you're rocking vuln weapons, and double-especially if you're rocking double-vuln weapons (which, for some builds, are easily available through a quest). Think I used to solo them with flee/hide/knife, vuln weapons, and bartering easily-obtained gear for heals at the Arkham healer.

So given you don't like the coral head mobs, what would you say are the level gaps (good and evil) for water ranking?