I'm yet to encounter any dragons in Icewind Dale II yet other than the very poorly used White Dragons, so I could not say for sure if its possible or impossible to effect them with Harm.
I'm guessing though that it should largely depend on the magic resistance of the mighty leviathan in question. Using spells like Lower Reistance, Greator Malison first however may help your cause...
Although, the tactic going toe to err, claw with a dragon with my cleric isn't something which will be high on my priority list when I get to where
you are!
I'll probably go for the more traditional chop-lizard method.
