Hi-Fi RUSH

You first will be able to use Macaron as a partner in Chapter 4 "Track 4: Less Budget, More Problems". You will come across several 4 legged spider robots and robots with a sword and shield, use Macaron on them and break that blue armor shield

Macaron is unlocked as a partner on Track 4: Less Budget, More Problems. He gives you the ability to shatter Red Z-Shielding on walls and enemy robots in combat. Macaron works in the same way as Peppermint does, pressing during combat will initiate a shield breaking attack. You can press to switch between Peppermint & Macaron then use their abilities.

To unlock the achievement you need to break 10 Red Shields from enemies during combat. This can't be unlocked in the training room unfortunately.