Shards are rather uncommon in normal play, and I'm hoping it won't be too long before I get my hands on bonus characters. Looking for things to do in-game to give me a slightly better chance at random shards. Thanks in advance.
Chaos Shine is the best way to get shards, the bosses in there drop them when you kill them. Bosses are random and their "set of 3 items" is random (seemingly) so if you're looking for a specific color shard just spam Dark Notes until you find one with a boss that drops that color and repeat that one until you unlock the character.
I'm not sure if this is the "best way" since I haven't played intensely and I've only unlocked one bonus character so far, but using the Chaos Shrine, the first stage I got had a boss that always dropped a yellow shard, so as long as I kept doing well in both the stages I was able to get that one repeatedly until I had 8. I got some extra yellows earlier from getting RM points, but that was way more random and slow.
Doing other Dark Notes has also yielded other shard colors, and the game is nice enough to tell you the loot list once you've obtained one so you don't have to remember which stage to play.
At level 40, characters get an "ulitmate" available. Cloud gets Omnislash. Tidus gets Blitz Ace, etc. These abilities do heavy damage and only activate when a boss appaears. I have three party members equipped with their, and alot of the times the boss falls as soon as he enters. They dont have a use limit. This is helps to get 2 or 3 bosses done per round, and increase your shard gain.