Necromancy

There is no "effective" way, it either procs or it doesn't.
It'd be good with someone like Anne & Mary or Hijikata so they can (possibly) get off a couple hard hitting NPs.

Use in on whoever needs it. Like when you need a chance to survive while soloing. It's quite bad on Herc (or anyone else really) since he has a better alternative (bond 10 CE) but if you're ready to sacrifice years of luck to clear a hard quest then go for it.

Hm... Maybe someone like Cu, Cu Alter or Nero? Those three are already pretty hard to kill so (and they don't have a Guts Bond CE like Herc), unless you need the CE slot for something else, it just might help them last a bit longer. Maybe long enough for their survival skills to hit their cooldown if you're lucky.

As others have said, there is not really a "efficient" way to use it since it either procs or it doesn't, but there will be those times you are extremely lucky and it procs multiple times in a row. IMO its best use is if you don't have a strong anchor servant leveled yet (like cu and herc) but need someone to clean up as a "just in case"

I dont think there is such efficient way to use this CE. It just purely rely on your luck. I have see somebody used it for ~7 continuous guts of solo a challenge quest. But it just applied for a very specific circumstance. You can have it on some servants to enhance their survivability

I really can't recommend using it on units with guts built in.

You can't pop guaranteed guts if you run that, FYI.