My prediction is that the dom lady will do very well. She doesn't want to kill anyone, she just wants to brainwash them, so she could succeed and it wouldn't immediately end the plot. If the assassin lady tries to kill another main character, she's guaranteed to fail, since I doubt the author actually wants to kill off main characters in his light hearted ecchi comedy series. But brainwashing someone into being sexually submissive is a perfectly acceptable thing to do in an ecchi manga, so I think it'll work.