The topic regarding balance today is spell resistance/spell penetration. EH should not require ANY past life spell pen in order to have NORMAL chances of success in defeating spell resistance.
What is a NORMAL chance you might ask or, more likely, argue?
NORMAL spell pen in the case of Ehard could reasonably be considered to be 20(pure class)+4 (both spell pen feats)+2 (spell pen item)+1/quest lvl above 20. In other words, Impossible Demands, a level 21 quest on normal or 22 on hard would have non-boss spell resistance of 28.
Think it should be higher? Let's examine that and see what it looks like shall we? BTW, it is higher, it just shouldn't be.
First, let me elude to the above framework. Said character had no spell pen oriented past lives. Can you guess why? Because they are supposed to be a benefit, not a requirement. Making them the standard makes them a requirement which is garbage, elitist and exclusionary.
Second, note they invested reasonably in feats and in gear. Wanna require imp shattermantle? Bad idea, making it baseline is the death of choice..other than perhaps choosing to play another game. Remember that the above toon was pure class? Is that the only one that should be viable for said purpose? I would have to say no, if you answer was yes it seems you want a game that doesn't allow multiclass.
Shattermantle should be there to fill that gap, don't count it in the calculation. Pure casters won't need it and wouldn't hit with their weapon anyway in most cases, other casters will hit and likewise will need it. Simple right?
Third, 1 per spell lvl means if you do it at lvl it is at the described threshold, you do it early it is harder, later, easier.
Finally, some will argue that it is somehow "supposed to be" harder. Let's start this with many things are supposed to be on both sides that are not and end with, put that mess in elite.
That is all.