Fight against Santa have little to do with luck - his girl's stats are fixed on each level. What are the stats of your event girls? Do you train her or not? First Santa girl's stats are 6.3k/6.3k/6.6k. So you only need to train your girl for 2 days to get to 10k+ in first two stats to be able to complete the battle. If you still can't finish first battle, it's not about luck, you are just doing something wrong.