Good luck for building your team. A good source of R-medals (for Centies and old 6*) is Danchou Tickets. You can get them with luck on gogo daily roulette, for 100 D-medals in the shop or after the 3rd and 7th roll on event gachas. The 3rd gacha D-ticket is the easier to get. It may be wise to look for extra ressources in the gacha (Danchou tickets, Rainbow shards, etc.) more than actual 6* from this gacha.

Take your time to build a second, third and fourth team. Getting to the end too fast is not fun :/
I still recommand one evade+intercept team somewhere. It makes really long fights but nothing can beat it (as long as it don't have accuracy buff or way too much HP).

Rainbow Pot memory is better than most of 5* memories for long fights or for Castle. It's really easy to get, to max and gives a nice +75% ATK for all with five of them fully unlocked.