This guide mentions looting, ability, pets, mastery sets, etc. in TROUBLESHOOTER: Abandoned Children.
Level and Character
There is loot that drops from enemies and chests with legendary gear dropping only from bosses, legendary and epic grade Tools (accesories) are what you’ll mostly be aiming for as these are useful for all levels.
Progession is divided into character level and class level, class levels make the character more efficient when fighting enemies of the same class and gives them masteries tied to the class as it levels, the combat buff class level gives stacks together for that character so maxing out all of the classes on that characters helps them, plus it opens up more skills and masteries for them.
Character level effects the gear you can use and the number of slots open on your mastery board the higher your level the more slots you have and the stronger gear you can use, advance classes also increase you mastery boards slots but each advance class has it’s own preset number of slots it gives you so you need to use other methods to increase them further.
There are a few other sources of progression too:
Masteries: Your XP will give you slots, but you need something to put in them! Levelling up will provide powerful class masteries, but you can also obtain them from defeating enemies (which can get you almost any mastery an enemy is using) or making certain choices in story missions (which you can repeat so don’t worry about missing one)
Mastery Sets: If you put the right four masteries on the same character this will produce a set bonus that can radically alter how the character plays. In principle, you can do this by guessing (or using a guide) but you can also find information on the sets in-game. Defeating a type of enemy enough times unlocks information on the sets used by that enemy.
Abilities: As your class level increases you’ll unlock new abilities (active things you can do on your turn), there are also some rare masteries and pieces of equipment that can add more. The abilities from leveling up need to be put into ability slots to be available – the number and type of slots you have depend on your class choice.
Pets: In the mid to late game you will obtain characters who can summon pets during the battle. To obtain better pets for the beastmaster you need to tame them during the battle, to obtain better pets for the engineer you need to acquire parts to build them out of.