make sure to make your daily tasks every day. aswell as killing your weekly bosses every week. also dont waste gems for resin. only use them for pulling. inside the shop system you can spend a special currency on 4star characters. i'd not go for them since you eventually get them anyway. i'd go for extra pulls since you can trade 5 of them for an extra pull on the currect banner. also stop pulling on a banner once you got the 5 star character constellation doesn't help that much. better get the next 5star character!
Never forget to grind your weekly bosses. Also, if you're having a lot of trouble with certain enemies, make sure you're paying attention to your elemental effects. Personally I find the game easy/casual enough without paying too much attention, but the elemental effects do make a difference when you're in a tight spot.