which Assassin?

From what I know, I think Camilla is a better assassin, again from what I know (which is not much tbh).
And I think that Scathach is a better crit star generator. Hope I helped a little.

Shishou generates more stars from her base hits. Plus her np generates tons if stars if used against 3 targets.

Carmilla does more ST damage particularly against females. Carmilla also makes up for her star gen deficit post strengthening when she gets her third skill.

So it's about even just Shishou's doesn't rely on a skill

Scathach has 3 quick cards , presence concealment E and also equipping her with a max limit broken opportunity CE can give around 20 starts per turn.