Got her after spending about 120 kobans (on Nutaku version). Not exactly sure after how much because it didn't show me any popup when I got her, so I overshot until I noticed she was gone from the rewards…

Just getting to fight the troll for her in the first place was quite a feat, though. The game has been completely broken for me ever since they gave out the free monthly card. It's not just about getting 503 errors every now and then…

Half of the buttons in the UI simply don't work anymore! It's like they don't have any links or event handlers specified on them. The following things I am unable to click:
Main menu:
- "Follow us"
- Buying energy or fighting points from main menu.
- Latest news
- "Thank you"
- Monthly card (it actually has been saying "Last day of the monthly card" since the first day I got it, and behind that is another text that says "Expires in 0 days")

Harem:
- Spending money to upgrade a girl does not work.
- The upgrade screen only shows the image, but no actual text for the scenes.

World overview:
- The big arrows to change between the left and right part of the world does not work.

The last point means that I was almost prevented from participating in the event in the first place.
Thankfully the game loads all locations when you go into that world screen and the arrows just change which part is visible. So I was able to modify the CSS style of the location buttons from world 1 to move the world into my visible screen, so I could click on it to enter it.


If this free monthly card promotion was meant to encourage me to spend money on the game, it actually achieved the opposite, since the game became pretty much unplayable since they activated it.