In order to earn this achievement, you must defeat three-hundred enemies with combination attacks.
For starters, a combination attack is an assault happening from you and the Majin at the same time. In other words, you both are finishing off a foe at the same time.
You and the Majin will have to hit an enemy together until a bar that's not full will come alive, on the bottom of the screen, urging you to hit the B button. DO NOT fill the bar all the way; that counts as a finish attack. Instead, just concentrate on getting plenty of hits in, from both, until the B button comes on screen.
You should be earning your combination kills throughout your playthrough. Hopefully, by the time you are done with this game, you should have the achievement. If not, the best spot to earn them would be in either Arkela Castle - Courtyard or Arkela Castle - Hall. If you need to respawn the enemies, just go to a Majin statue, pretend like you're going to change your costume, then exit out.
I earned my final 300th kill while farming for the achievement as the Arkela Hall - Courtyard, since it was easier for me to focus on one or two opponents at a time and it didn't take me long to get the achievement from that point on. Time-consuming? Yes. But worth it? Oh yes it is!
Have fun and good luck!