The King of Fighters XII

Make sure the settings are on Easy and 99 round time limit. Timeouts do not count.

Use simple Mode with Leona Heidern. Use the LP version in X-Calibur which is DF+HP while in mid-air. Use the super special when possible, viewable in command list. D,DB,B+LP or HP

Timed just right as they are getting up you can trap them in the X-Calibur loop, but not always.

Have a second controller ready in case you're about to lose a round or time out.

Against Robert Garcia, Mature, and Athena Asamiya you must make use of your earing attack which is B+HK

Acquired by beating Arcade mode with Leona without losing a round.

To be more specific, you must put Leona in your team and choose her to start 1st. Then you cannot lose a round at all or choose "retry" after you finish the stage. As long as you defeat all 15 opponents with Leona, you will unlock this achievement at the end of arcade mode.