I think it's possible that a lot of these issues are addressed by putting hp thresholds on spells...matches a lot of narratives as well (Having to weaken your powerful enemy before the spell can snare him or some such).
Personally, I preferred 13A's Escalation Die.
You
can take the risk of going nova on the first round....but it's a big risk and a bunch of your most powerful stuff probably won't land. Or, you can wait to deploy the big guns for when you have the big bonuses...when things will be less efficient and an AoE being party-unfriendly is actually a concern.
I see that as being both more engaging and more tactical than "ope, sorry, the boss had one hundred
five hit points, so I'm afraid your
disintegrate did nothing, better luck next time!" Even outside of that relatively pointed example, it's just kinda dull to have entirely-arbitrary integer "you must be at least this dead to Save or Die" thresholds, while the Escalation Die actually acts as an
incentive to change behavior--on multiple axes, even. If you want the die to increase, you have to actually
escalate, for example; you can't just turtle up and pop in and out of Total Cover to accomplish your goals.