Chained Echoes

You will gain your Sky Armor toward the end of Act I in the story. Sky Armor weapons gain proficiency through battle. SP earned at the end of battle will go toward the equipped weapons you have, meaning you can work on two weapons at the same time (you'll also get very small progress to unequipped weapons as well). Once you hit Rank 8 with the two equipped weapons, be sure to swap them out for new weapons and continue to do so until all nine weapons have been maxed out. You get more SP from defeating large enemies in your Sky Armor rather than beating up on enemies you'd normally fight at your normal human size, so don't bother with that.