DEATH EFFECTS
Death effects and other "instant kill" effects like petrification have never been a TRULY balanced part of the game. There's too much randomness to it. A high level character could die by low level monster instantly via natural 1 in the first round of combat. So I like to have a few house rules for players in my games.

Several special immunities are granted while you have 50% or more of your max HP:

• You can't be compelled to commit suicide.
• Death effects and banishment stun for 1 round and deal damage instead of killing/banishing. The damage is 1d6/2 CL or whatever the ability dictates (whichever is more).
• Ability damage/drain and level drain can't drop a score/level below half the normal amount.
• Disabling effects cannot affect you for more than 1 round at a time. A "disabling" effect is anything that causes the target to be unable to take their normal actions (confused, dazed, dominated, frightened, paralyzed, staggered, stunned, panicked, unconscious).

*These immunities do not apply against a foe with a CR that's 4 or more points above your APL (which you should never be forced to face unless you go out of your way to pick fights with the wrong NPCs/monsters).

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COUP DE GRACE
Below are some new coup de grace options.

NONLETHAL
When dealing a coup de grace with a nonlethal attack, the target is knocked unconscious instead of killed upon a failed Fortitude saving throw.

PINNED
A pinned target is susceptible to coup de grace attempts except that they do not automatically hit or automatically crit. A normal attack roll is still necessary. A pinned character that is prone is at an effective -10 AC (-2 AC from grapple Dex penalty, -4 AC from pinned, -4 AC against melee attacks from prone).

DC 10 + DAMAGE
Because saving throw progression is not proportional to damage and hit point progression, "DC10+damage" has never made any sense. Instead, whenever this would normally be the case for any DC, change the DC to 10 + 1/2 HD + attacking ability modifier instead. If using a weapon, the attacking ability modifier is Strength or Dexterity (depending whether they're using Weapon Finesse or not). If attacking with a special attack, use the same DC as the special attack.

ABILITY DAMAGE, ABILITY DRAIN and LEVEL DRAIN
A creature capable of dealing ability damage, ability drain or level drain can perform a coup de grace with these special attacks instead of a normal weapon attack (though they cannot crit with them unless noted otherwise). If your ability damage/drain does not affect CON, then it is a nonlethal coup de grace.

DAMAGE NOT IMMUNITY
Death effects can also be altered to ability damage or drain or level drain instead of instant death. Take the CR of the creature that has the ability and cut that in half then add 2 or some other condition of equivalent value. That is the ability damage they should deal with the death effect.

Example:

Medusa CR7 8d10 HD Petrifying Gaze (Su): 1d4 Dexterity damage + slowed as if by the Slow spell for 8 rounds (DC16 Fortitude for half damage and negate slow effect). Any creature reduced to 0 Dex while affected by a Petrifying Gaze turns to stone as normal.


Not "powerful" enough?

Make the ability usable only once every 1d4 rounds and double the damage/add more conditions:

2d4 Dex damage + slowed for 8 rounds + dazed for 1 round

Note how this is still incredibly useful for a medusa since they are archers. And even if they weren't, the daze and slow effects do a great job of "dividing and conquering" which gives even a frail monster a decent fight (assuming that their DC isn't so low that even the low save characters pass easily).