Hot Take: Killing Blow is bad if you're using Amalgam Organ Shatter.
Hotter Take: What if you could build up to 12x combo count by blocking, and then spend 10% for a heavy attack whenever you swap back to the Tenet Livia?
Hottest Take: Reflex Guard is good bad good, actually.
These are probably all bad ideas because blocking is currently extremely buggy, both in the 'you randomly get hit' and the 'doesn't build combo count sometimes' ways, and thus abusing the Livia's passive to build combo count is incredibly unreliable. If for some reason you want to do this anyway, here's a build for you!
Please use a very tanky warframe, and Primary and/or Secondary Dexterity Arcanes to maintain combo when not holstered.
Ideal Riven includes some amount of +Range, +Combo Count Chance, +Heavy Attack Efficiency - any one of these stats allows it to seamlessly replace a mod, two allow it to replace two mods(though, be warned - Heat damage as a bonus is better after replacing Focus Energy, unless the Riven also includes Electric or Cold) and free up a slot, and three would be wildly unlikely but incredibly good.