[WraithLord]

You're right of course but usually what happens is that first turn you throw a bunch of EAs at someone he'll be caught unprepared and the earth elementals will kill his Pans.
2nd time around he will be prepared so you come with something else. For example, casting 3EAs on the same Pan will kill him unless you put 15 or more N gems on him (scripted as you described). But its possible to defend against that as well of course - Its like a game of chess in which each side tries to best guess the other's plans and then counter them.

The rule thumb is everything can be assassinated. The question is what's the cost of the assassination?- Is it worth the price or not?- clearly that depends on the context.