This update adds the mission bosses to Training mode. Individual bosses will be unlocked only after clearing them in Arcade mode, either by destroying or timing out.
Bosses will start at the highest mission rank cap, while the focus counter is set to 1000. Training mode bosses represent the hardest possible form they can take, meaning that their versions in Arcade mode is most likely less difficulty since players will normally not be at max rank unless they are already doing an advanced skill scoring run.
Use Training mode's unlimited armor to find a strategy that works without worrying about getting hit, or try changing the player power level to see if a lower level is also doable.