Originally Posted by kabosu

you can use the bond crystals to buy medals and it's not in the game yet but you'll be able to buy gacha tickets with those medals which apparently give you 30 rainbow medals when used

You can purchase Leader Medals with Bond Crystals, not rainbow.

Rainbow Medals, outside of possible daily deals (today's is the first that offered them), they're obtained when you roll 5*/6* dupes in the gacha. It will take a long, long, long time before you get the 300 needed for one 6* unit.