I'd like to see rock giants immune (or highly resistant) to piercing and slashing damage, as well as most fire or lighting based spells. You should need crushing damage to hurt them or very hot fire, etc.
They don't really count as cool, but insect swarms should be highly immune to physical damage; 3 swings of a sword shouldn't be able to kill a few dozen or hundred insects. They should be vulnerable to most magical attacks that have any kind of area effect (even secondary effects, like heat from a fireball) as well as torches (perhaps laced with incense). Small swarms shouldn't be able to do a lot of physical damage (though could be very distracting, lowering you offensive and defensive capabilities) but different insects could use different types of poison.
I don't want to have to carry a dozen weapons with me, but needing 2 or 3 different types to adapt to different opponents is reasonable and realistic.
