Bosses have base 145 Fire, Shadow, Frost, and Arcane resist, and 70 Nature resist because there's no curse to reduce nature resist.
Chain Lightning does not scale as an AoE because it is not an AoE; it is a single-target spell that bounces for reduced damage and it has the coefficient of a single-target spell. Chain Heal is the same. Here's an old spelldmg comparison chart.
