This is story based and cannot be missed. The very beginning of Guacamelee! 2 is actually a flashback to the end of Guacamelee! You will face off against Calaca after some brief dialogue and running up the stairs. Simply dodge his punches and score some counters and beware when he drops down to pound as it has an area effect. You'll beat him to the ground in no time before the Black Sun transforms him into ??? Calaca. Just use Y to throw the falling rocks at him while dodging and jumping over his attacks. Shortly after he will be defeated and the achievement will pop. This is WAY easier than in the first game!