KR - How to Solve the Guild Raid Problem (2016)
I explain it all in my video. What's important is filling all 9 dragon core slots available. There's a difference between the rewards from the Bronze, Silver, Gold and Platinum bosses but if you're replaying a boss over and over then the stat differences between the cores given from 35th and 1st are very small so it's no real disadvantage to only place in the bottom 25 each time.
I rambled a little bit in order to properly cover everything and drill in my points. Hopefully I've said enough to convince you.