Anything that can die should be killable. Now I am not saying that it should be easy to kill a foe. In fact it could very be near impossible to kill a foe, but if you have enough fire power to do so, or use some brilliant strategy, then the foe should be killable. I mean, take CoC for your example. How many high powered critters in that game can stand a full nuclear assault? Not many I believe. Of course, not many PCs would have nuclear weapons, but it does not change the fact that the critter needing such a weapon to kill it isn't killable.