The game it self is a big sandbox: u can upgrade ur lvl with all stats quite fast, upgrade lvl a little and go to another upgrade menu where u can choose with stat you want to be upgraded/get special abilities [for example, counter attack to 1 enemy that hits u last], explore the.... school to find equipment and weapons, buy a big variety of consumable items that can matter.
Even if u just want plot - upgrade lvl it's easy, don't use monsters abilities, skip ero scenes with your comrades.