To win in Arena and Leagues, you need to max your stats.
The most important thing to max is the counting stats next to your name on the market screen. So many players dont do this. Then, max your gear. Purple Gear is max gear and you get it from Epic Pachiinko (or koban bought Pachinkos). You want Single Stat gear that matches your affliation. (if you are hardcore, get hardcore gear). Super Sexy / multi stat gear is good too if you can get that, even though orange single stat gear is about even or very slightly better. Purple Gear is better than other gear for about 30-40 levels. So that's how often it needs to be replaced.
If you are Low level, Pachinko *10 is insanely good. The legendary piece of Equipment is worth more than the price of the Pachinko, and you could get (RNG) expensive money saving upgrade items.
The least important thing is the stats of your Alpha (and beta) girls. Your third girl rarely factors into the battle and you can use that slot to train your harem. A five star legendary girl is worth about 8 levels worth of stats over a Starter Girl. That's nothing if your gear and bought stats are not up to date. Nonethleless, You will eventually need to get a 5 star legendary girl as your alpha. Hoard at least 600 kobans for refills so when legendary days pops, you are assured to get the girl. However, until you get the 5 star alpha girl, the differences between even legendary three star regular Event and Pachinko girls and common 5 star Free girls is minor/meaningless. (Example, if you are Hardcore, you will get Bunny and Fairy for free relatively quickly and both are about as good as/better than much more expensive girls). Don't break the bank spending money on the wrong things like upgrading expensive girls that will not be your alpha in the endgame.