There are two different sex scenes; the first is with the unconscious grunt enemies (axe, shield) and the second with the big brute. The big brute gives more ninjutsu bar regeneration with button spam. Essentially, killing just about any enemy with the regular assassination technique gives a chance for them to have erect penises for the player to use as energy batteries.
The brute can be beaten with regular attacks; simply dash behind him and strike with regular attacks. When his HP goes to zero you can finish him off with the assassination technique.
For the shield users, I find being one person's width away from them when they're unawares gives the best timing to dislodge their shield. Too far and it won't work, and at very close range she'll jump past the target.
For the two projectiles with limited quantities, the Kunai is the standard damage-dealing projectile while the smoke bomb (5 uses) blinds the affected target making them more or less sitting ducks for assassination. Not great in crowds, as doing the technique leaves the player vulnerable afterwards bad luck means free damage for the enemy.
2D dot version of Sekiro, so get used to sneaking around visual cones. The stats do seem to emphasise staying undetected, but the game is still too early so no idea what reward players get for doing stages like a shadow.