What it says on the tin, really.
I'm trying to get a better Critical Multiplier on a weapon.
Now then, certain points to address right off.
Point (A)
Whatever weapon used would start with a x4
Therefor, Kaorti, which wouldn't be allowed anyway, is not relevant.
Point (B)
No AEG, or most other third-party books. Dragon is okay, as is a lot of homebrew.
So no "Serrated", if that's what it was called.
Point (C)
Dragondoom only applies against dragons, and is too many feats to be worth it.
Master Throwers Deadeye Shot does not apply, due to the melee nature of things.
So neither of those work.
One thing I might use:
Maiming is random, but it does net a statistical .5 increase to multiplier.
Weapon properties, whether mundane or magical, are best.
Spells are great.
Feats are okay.
Levels are... tolerable.