Swords against tentacles!
Another thought for rolling a deadly critical as Xoth has proposed. You could allow the proficiency bonus to be used on the second critical attempt to increase the critical range.
So a level 1 character with a proficiency bonus of +2 would have a deadly critical range of 18-20 instead of 20.
The hard work of the initial critical has already been done in the attack, so perhaps it’s only fair that the second critical attempt has this modified chance?
Then again there’s always the other methods I suggested where the DC is more easily modified to hit the sweet spot
- The Con save against 10 or half damage whichever is higher, or just make it challenging DC15
- Str/Dex roll to beat the targets constitution score.
- Or go the other way and really mix up the odds by making it an opposed roll of attackers Str or Dex vs the targets Dex or Constitution.
I think the original idea is fine as it is (roll two crits, target goes to zero hp).
As you have pointed out, there are already several options that can increase the crit range:
- The Fighter/Champion's Improved Critical
- The Rogue/Assassin's auto-crit against surprised targets
- The Conqueror's Battle Cry that turns all hits into crits