Mega Man ZX Advent/Medals

Medals can be obtained from bosses that you fight, both the Pseudoroids and the Mega Men. these can be obtained by defeating the boss without sustaining damage. The difficulty of this varies depending on your skill and the boss you fight. There are 24 medals in all, which are available from every boss you fight. These medals are needed to unlock a hidden model after you complete the game.

If you want a hint on how to gain all of them, go look on youtube or something.