Board Thread:Questions and Answers/@comment-25216437-20140724151307/@comment-25336507-20140724153348

to max skill level, you need a card or mags with the same skill. But to increase the level of the card, people usually farm a 'feeder' card in chapter 2-2-1 and fuse them.

you can read the detail here:

Skill leveling Tips

Fusion Tips