All boss battle events are unlocked automatically whether you win or lose. Just go through the story and you will get all of them.
The mercenary guy in the guild can be hired with Karen's body. 5 events, lust 7 12 22 30.
Before Elysia joins (or after she left?), go to the bath in the second town.
When Elysia is in the party, go to the bath.
When both girl is naked (see below), go to the bath.
Protect Elysia's chastity and get Karen's raped before going to the village.
Or keep both of them safe until going to the village. (you can only get this scene, or the scene above, or neither in one playthrough)
Sleep in the inn in the second or third town, and choose 2nd (Karen) or 3rd choice (Elysia) events. 6 events for each girl.
After first event in the third town guild basement, you can work at the brothel (Karen, lust 20, 25, 35), and the pub (Elysia, lust 12, 20, 30). 4 events each. You need to sleep after each event.
When lust is 25 or greater, go to brothel second floor to prostitute the girl.
Lose a fight in the arena. Then win the next 3 fights (losing here leads to game over, which means the scenes won't be saved).
Buy the saw from the merchant in 2nd town (5000 G). When Karen has 50 lust or greater, it can be used to strip the girls to enter the slum in 3rd town.