Are you the character dealing the damage or the character suffering the damage?
Cause the former is just a Merciful weapon.
The latter is.....Troll-blooded 1st level only feat that requires Toughness as a pre-req.
And a few other means of gaining Regenerations.
There's the Delay Death spell. Doesn't change the damage type inflicted, but temporarily removes your -10hp cap on death to infinite until the spell expires. The spell lasts rounds, you gotta heal up before it ends or ya die anyways.
Higher level is Hide Life. Effectively the same as Delay Death, but instantaneous duration, thus permanent. So long as the body part you cut off is never found and destroyed. It's 9th level and has a 5k exp cost and takes 1 day to cast. Buy a scroll of it. And a scroll of Divine Insight & Wieldskill so you can just UMD it