You need wine and lust potion and then go to her at night and that's it.
Also, remember there is still 1 girl left in the party and we also need to 'remove' Leon and maybe Eris will help with that ;)
And yes I purposely made the girls in the game fall fast for the MC cus our MC (even after losing most of his power) is still a pure manifestation of lust, so making a newbie adventurer fall for him is easy. Of course, there will be some girls harder to get (like Dianna) and I want to make a lot of girls (And I really mean A LOT OF) in the game so it's normal that getting some will be easy and others will be harder. :D