Cleric soloing is among the slowest in the game, at about 7 hours per level root-nuking undead. If you outfit your cleric with really expensive gear (i.e. fungi, haste item, poison wind censer (this latter isn't very expensive but you can't pull off a melee cleric very effectively without a fungi)), you can speed it up especially at lower levels but it's still surprisingly effective at high levels (I'm in my mid-30s doing about 3-4 hours a level). Root-nuking, apart from being slow as fuck, is also boring as fuck (some people would also find cleric grouping boring and one-dimensional, though I think it's pretty fun). So I'd say, try to find groups, especially small groups as mentioned above, but if that fails or you don't have enough time that particular play session, go ahead and solo a bit. Swish has a nice guide here:
http://wiki.project1999.com/Swish%27...ric_SOLO_guide