Yeah, the problem with Woundflame is that it usually kills anything it hits, so there's no one to spread the plague. It's worth trying against heavily protected units (Ulm, LA Man) or big units (Niefelheim, Lobo guards/shamblers) but for smaller armies it may be better to just cast plague with a D4 caster.
-Max
Edit: that is, smaller in terms of individual unit hit points. Plague works wonders against large armies. Here's two necromancers and some meat shields against 1000 R'lyeh chaff in a SP game:
. That's far better than I would have expected from any map killer. You still have to survive long enough for the Plague to take effect, but combined with Army of Lead it's great. Of course there's still the matter of "friendly fire" plague casualties. You can either kamikaze your units, use undead units (which are immune to plague), or get a small amount of regen through a Nature bless or Mass Regeneration spell.