NOTE: This is only based from MY EXPERIENCE.
How?
1. Enter Xenia Frontier 1 star difficulty.
2. On the jump stage you can use any char but I recommend a DA since passing the area is way too easy.
3. Take the left route (Octus route).
4. Pwn the boss ^_^
Reason: There are 2 bosses in xenia frontier, and you'll encounter them twice, giving a chance to drop 2 red cores and 2 shining cores.
P.S. I made a wrong spelling for shining cores in the thread title, sorry.