Diviner hardest? No way. They only miss out on conjuration/summoning, but there are still other summons that use other schools of magic.
I'd say the hardest class to solo would be a single class druid (no kit). Totemic are pretty easy to solo, Shapeshifters rock at level 13 and up, and Avengers are not too bad; but with no kit definitely a Druid is the hardest.
I soloed a Jester, pretty fun actually.
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"This place will be your tomb."