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[tanknspank]

Necromancers are very strong solo. They are evil, however, so you will have to rely on your invisibility to get around. Merchants that deal with them are more sparse as well.

Druids can solo well through a few different methods depending on your tastes, location and risk-aversion. Using charmed animals, quad-kiting, root-and-dot, etc.

Clerics, shamans, magicians, enchanters would be other choices I've heard suggested tho I haven't played them myself.