Concentrated doses.Sufficient alchemical skill allows a character to distill extremely powerful concentrates from existing poisons, raising the DC of the poison.
DC+5 = +1 DC to poison save, and the new DC for crafting is the old DC+5
Failure ruins the dose entirely.
Example:
Craft DC 20
Contact DC 10
If I make a DC25 craft check, I can distill this into a DC11 poison.
Craft DC25
Contact DC 11
One other change to the system: Immunity is a +10 untyped bonus to saves, not invulnerability.