Bosses have an armor gauge. When their armor gauge is intact, Bosses will take reduced damage. Use regular auto attacks against the Boss to damage their armor. When the armor is completely gone, the Boss will take 30% more damage. You are now free to use all your skills. Bosses will use very heavy attacks against you. Red indicators will appear when a Boss is about to launch a heavy attack. Retreat from the marked zone as fast as possible. If you get into trouble, use your evasive skill to move around faster.
Stars
Each major level has three gold stars that can be earned by taking minimal damage and completing them as fast as possible. The more gold stars you earn get in a chapter, the better the rewards that are given. Tap the "Chest" button at the lower left of the chapter stage selection screen. You can open a tier 1, 2, or 3 chest depending on how many stars you have earned at that point in the chapter.
Upgrades
There are two main methods of upgrading your character. Enhance each equipment slot to get more stats boosts for your character. Enhancement is independent of your actual equipment. If you change gear, that specific slot's enhancement progress is retained. After you enhance all the slots to their maximum limit, you can perform a Break Equipment Limit to even further enhance your gear. You can learn new skills and upgrade existing ones after reaching a high enough level.