This is because 100% spell resistance does not prevent 100% of spell damage. For most damage-doing spells, resisting a spell only halves the damage done, and magic resistance is the chance of resisting a spell. So even "resists all spells" effectively means that all spells necessarily do half damage. If you're doing half damage with a built-up Archmage of something, that's still going to be more than respectable.
This. The things that won't ever work are spells that "create" an effect, such as Blind, Chaos, or Shrink. Area damage spells also succeed, though as with direct damage spells, there's a chance for half damage due to resistance.
