In general, there isn't a single weakest faction. Life is strong against Death, Death is weak against Life and Nature, etc.
Against Nature: Plague. If the Druid doesn't know Life magic, he's done for in a few turns. :( + if the Necromancer knows Order magic. Banish, Hypnotize, Sacrifice a hypnotized creature. Against Life: Curse, Remove Beneficial Effects, Hand of Death, Wave of Death, Plague, and so on + raising undead.
