1 (Not so important when you start but pretty essential as you progress) - ALWAYS remember your inquiry code (Is on the upper-right corner of the menu,or in settings can be seen).Without it,you can't recover your data if it is lost (You can take a screenshot so that you always have your inquiry in a safe place).
2.Get Best quality (Superior) treasures - May be time-consuming,but it is essential.Treasures can make things much easier,and some stages are nearly impossible without them.Also it can help you to gather XP and upgrade your cats.
3.Use Catfood efficiently - Catfood is a hard-to-get resource (Unless you have a lot of free time to watch ads),so it isn't good to waste it in stuff such as energy refills or continues.A good thing to do with catfood is to store it for 11-roll guarantee gachas (The best way to get ubers).
4.Use your XP for the essential upgrades first - I recommend to start by unlocking cats you have locked,and upgrading Cat,Tank,and Bird Cat first.Also is great to upgrade Cat Study and Cat Energy,as they'll help you to proceed faster.Cat Efficiency is also good,but don't forget to upgrade Cat Wallet too,or else you'll not be able to reach MAX level.
5.Don't sell Gacha units - Gacha units are good to keep,no matter their rarity,as they can be very helpful in some stages.Even if you already have them,it is recommended to keep them (Unless it is a duplicate Balaluga :P),since more levels mean more strength.It also helps to get user ranks.