I tried this when it first came to Nutaku, and missed the 2nd free champion for 50 levels in 24 hours (I got stalled at the level 30 or 35 boss, and didn't realize you had to level all the useless girls in order to get the reset to pop up). I gave up steady play after about a week, because promoting costs 250 crowns each time, you needed 11 promotions per girl, and free crowns were few and far between. So I just logged in once a week or so, spent money on the new girls, pushed to the next boss, and let it idle, so I could see what happened at higher levels.

Recently, I noticed the contest. That kept my interest for about 4 days, before it became popular (rank 10 for 2 promotions the first day,, rank 47 for 1 promotion the next, and from then on I was unable to maintain a rank above 50, so no reward)
For me, the most frustrating thing about it is the girls are only useful from about level 6 to level 35 (raising the 2nd highest girl from level 5 to 6 gives more DPS/gold than raising the best girl to level 2, but by around level 35, the gold cost to level makes the DPS increase trivial). But you have skills that you are supposed to reach level 50, 75, 100, 250, 500(!!!) to buy.