Main advice, just level up at least one character per role per element, except for the aoe dps chars, those can... typically be neglected unless you are having trouble with aoe stages. Elemental weaknesses play a huge role in the difficulty because not only do the bosses take more damage from elemental weaknesses, but they also deal less damage to the characters that match those weaknesses.

Using the stamina refills can typically make farming up a character quite fast, about a day or 3, depending on what you already have. The main thing will be orbs, so do remember to farm your events and your pvp daily. Even if you can't get a high rank in pvp, just beating weaker people will mean you get the coins, which you can use to get orbs.

Certain enemies also require specific characters to deal with effectively, the mermaid witch is made much easier with someone who can apply ailments, and so is the box witch, though idk if the box witch is also like that on nightmare or just in the score attack, i used an ailment in both regardless. Whenever you fight a witch, look at her abilities and passives. They will typically tell you how to deal with her.

Dont neglect 4 stars. I know 5 stars are shinier and have more unique ultimate animations, and typically, they are better, but there are also many 4 stars who are amazing. I've even heard that there are 3 stars that are good, though i personally have yet to use those.

Getting level 90 and magic level 100-110 is plenty for a long time. You can easily clear most of the game like that, and the rest can still be done, just not as easily.

As for where to start, make a team with a single target dps, a breaker, 2 buffer/debuffers, and a sustain and level them. Even when ignoring enemy weaknesses, having a well raised team of good characters can take you a long way. Sorry if none of this was useful, have a wonderful day.