El Shaddai: Ascension of the Metatron

This ability will become available during your fight with Sariel at the end of his level of the tower after he changes forms. Simply hold left and right bumper at the same time when you hear Uriel's voice say "Let me offer my aid." - you will also see a visual cue in the form of flames erupting from your shoulders/back when overboost is available, which triggers after dealing/taking certain amounts of damage.