How do I get S-rank for COs?

[Achelousaurus]

Plenty COs require to get S-rank in a certain time and I still don't know how to reliably get S-rank.

I heard things like kill everything that spawns, kill all nemies you see for the first 3 minutes, ten rush to the boss or just kill the big enemy groups at the end of area 1 and 2.
Neither of these is any reliable and I get plenty A ranks.

Does anyone know the actual game mechanics behind it?
I heard it's about an enemy kill counter, but I can't find it, is it entirely hidden or can you somehow see or estimate the value?

[THRINE4]

Kill everything that spawns is the safest way to secure S-rank. Killing around 60 mobs should be close to the S-rank cap, and even then I suggest if you got the time, kill at least some more, because it's better to spend another few minutes of extra killing and secure a S-rank, instead of getting A-rank and being forced to redo again. Dying and later getting revived is fine, but dying and returning to campship will cause points to be lost, meaning no S-rank.

[Achelousaurus]

Thanks, will try that.

[LordKaiser]

I think that 3 e-codes work the same way but I'm not sure.

[Achelousaurus]

Killing everything that spawns seems to do the trick, 3 times in a row that got me S-rank.

Something REALLY weird happened just now though.

3 minutes into SH Coast exploration I failed both "Are you a top Braver now?" and "Superior Katana Training".
I was doing it with Azanami, only using a Katana and no one died, both at 50%+ HP the whole time.

Then a bunch of Tyraluda or Strahda spawned a bit away, I could only see that they were flying darkers.

I turned around (5 seconds later) and walked 3 steps back to the telepipe - FAIL!
How the hell did I fail, nothing whatsoever happened?

I didn't get a voice exclaiming I failed, there was no ET, but I just got 2 system messages for no reason I can imagine.