When Emil and Marta both have the skill "Special" equipped, they will sometimes do a special unison attack called "Arse Nova". This happens randomly at times, however if certain conditions are met, it will always happen. The key is in the elemental grid at the bottom left corner of the screen. When the grid shows equal parts dark and light, press C when the unison attack gauge is over half full as either Emil or Marta to do Ase Nova. If the Field Element is Light, do one base Dark-element attack (for example, Demon Fang with the skill "Tenebre" equipped) then do the unison. If the Field Element is Dark, do a weak Light attack (for example, Savage Reaper with the skill "Lumin" equipped) then perform the unison. Arse Nova is relatively powerful, and usually gets anywhere from 15 to 30 hits in as a part of a combo.