Extra Extra Lives

Must be done in Extra Life Mode.  This is a cumulative total - you don't need 20 in your possession at the same time.

This can be farmed in mission 2 (Exultia) - at the very start drop down the left in front of you and there's an extra life there.  Quit to main menu -> Mission Select -> restart mission -> rinse & repeat.   NB you must have already completed mission 2 for this to be available.