Infinite Guitars

You can earn this achievement on the area boss of Cebu (floating island 4). Play through the level and eventually you'll get tossed in confinement and released by Ru, who will join your party to head off and fight Bala and the Beetle Mech. On the zone outside the boss fight, you'll see a couple injured people and two checkpoint guitars way too close to each other. Make sure you do a manual save on a free space in case you're not able to get this achievement on your first try.**

Once the first part of combat with the mech has started, you'll have to run around a little bit and dodge drills and missiles as you've done in the past with other boss fights. Theory says this should be pretty simple: roll past a column of drills, dodge a couple missiles targeting you as you approach the Mech's 'arm', and instead of bashing to swing your guitar at it, take your time and hold for an extra second to 'wind up' your swing and release it on rhythm with the music. JJ will blink white to give you a sense of what the rhythm is to help you land your 'perfect guitar hit'. Unfortunately there's no real indicator that you've done it right, other than your charge increasing by 1. Do this a couple times then run out and around to avoid a barrage of missiles fired by nearby turrets. (In my opinion this part of combat is the best music-synced part of the game.) Wash and repeat.

If you finish the rest of the battle session (the actual battle and clash) and the achievement doesn't pop, you may need to reload your save and try it again.

** You will have another opportunity to get this achievement when you head back to Mikati and battle all the mechs once again, though the ongoing missiles that target you seem to be more frequent.