Event looks like Xmas so far. Scroll up a lot to read about it.

If the mechanics did not change, you have a nonzero chance of winning, no matter your stats. The chance should be at minimum 1% and if you have stamina higher than 1000 it should be stamina/1000 in %. At the Xmas event I could not confirm this myself (not enough trys), and it seems, the formula was previously visible on client side for optics. Since then things could have changed.

The event outfits are cosmetic anyway. Grab a 70% outfit for your favorite girl and your highest girl you cannot afford clothing. After that, try your luck after you have risen in ranked challenges to your max rank.

And no, event girls can't be leveled up. Only trained. So you are stuck with 20 stamina. So is Mistlehoe, the elf from Xmas. That makes 10302 stat points per day of training. So 10k per 25 days makes about 75k * 18% outfit. With booster items, expect stats of 100k, 100k, 100k near the end of the event for free players. Or 150k 150k 0. This is a luck event anyway, as it seems.