Save or Die effects aren't great for monsters because there's too much riding on a single roll. They're not great for player-character powers because they invalidate the way most characters bring down enemies: hit point attrition. In 13th Age, we picked up a rule from Dave Hargrave's Arduin Adventure game. Save-or-die effects have a hit point cap. The death lord PC in our game uses the "Sever Spirit" power, which is an instant kill, but only against an opponent with 60 hit points or less. Likewise, my cleric's fear spell works only on enemies with 75 hit points or less. That way, other characters can contribute by knocking a monster's hit points down, and then one of us can finish it off with a power.