These problems are: Limited talents early on and screwed against an enemy with 100% immunity.
The easiest solution to this is to take an attack spell in another element, but this is offensive to min-maxers as they won't be stack +damage in that element.
The suggestion of moving the res pierce talents to slot 4 of the unlocked category limits what you can put in the category, as that means you only have 3 talent slots to fit in a decent range of offensive talents to get you going. This leaves no room for interesting mechanics, or even defensive/utility talents.
The alteration of the res pierce talents to all have the same extra resistance to self damage means that there would be 5 talents which do basically the same thing. This suggests unnecessary duplication.
So I've been thinking about a new category for Archmage that would start unlocked, and would be designed to assist with mono element builds.
The first talent is a basic attack spell which uses whatever damage type your staff is set to. Like a power Channel Staff. This would mean that single element builds would start with 2 attack spells, and would mean that the element specific one could be more interesting in shape or effect.
The second talent would be an instant buff that gives you +50% res pierce all for 1 turn.
No ideas for the next two talents.

As an aside, instead of making 5 talents that give resistance to self damage, why not just change Spellcraft to do that instead?