A couple things I have noticed since rolling a sorceror about monster immunities:
Hill Giant Hunters in Stormcleave are immune to fire, like fire giants, but not vulnerable to ice, unlike fire giants. Hybrids?
Clay golems. Jesus, clay golems. Immune to fire, ice, and force, and acid heals them. Don't know about lightning but how is animated clay impervious to most magic? Overheating clay will make it brittle and shatter it, freezing clay will shatter it, and acid tends to dissolve clay, since it is not a glazed ceramic.
And iron golems are immune to force. Who knew that being a golem made you impervious to... impact of some sort?