Burst mode
After level 4, three orange bars will appear under the health and mana bars. They will fill when you attack enemies. When ready, a glowing blue button will appear in your skills list. Tap it to start Burst mode. This will temporarily increase your attacks. Use it often, as it will refill quickly.
Skills
Depending on what slot your skills are in, they have other abilities.
Slot 1 skills are usually a type of evasive attack. Although their damage is low, they will move you from danger fast.
Slot 2 skills are small area of effect attacks that will give you super armor status while executed. In this state, you will be immune to becoming stunned when hit.
Slot 3 skills are unique skills for each character.
Slot 4 skills do massive amounts damage in wide area and break super armor. They can be used to interrupt a boss charging a big attack.
