I understand "targetable" this way. Needed 30 min for this (hopefully) bullet-proof definition.
A creature, weapon or permanent, which is nontargetable, is immune to all spells (with a lightning bolt ), active (or activated)skills of permanents, permanents with upkeep costs, weapons and creatures except these , which have
1) always exactly one target and
2) a target which is chosen without the red cross.
The spell , active skill and so on must be newer, than the immunity. Old spells, active ...(and so on) don´t lose their effect in most cases. Freeze is an exception (bug?).
I went through all cards, I think this rules descibes the effects of Quintessence and Enchant Artifact without exceptions ( until now )
Some explainations:
Voodoo Doll skill is passive. Weapons are not immune to it.
Shields have no active skill. Immortal creatures are not immune.
Animate weapon has one target without a cross. Weapons are not immune to it.
Untargetable creatures are immune to Rain of Fire, Pandemonium, since these cards don´t have exactly one target.
Chimera has no lightning bolt.
Nontargetable is a skill, so Luciferin doesn´t give bioluminescense to nontargetable creatures.
Ignite is the "active skill" of "Unstable Gas" so, untargetable creatures are immune to it.
An immortal creature can hatch, because it´s one target without a red cross.
Immortal creatures can´t drown in the water. Iundation has upkeep costs.