Best way of course would be to sell all your S.Rare
(★★★★★) for 5x and Rare+ (★★★★) for 2x per
card. But, if you are like me, I like to keep my cards as this is a
collecting card game ;)
What you could do though, is to keep your S.Rare cards
as they might come in handy every week for a ATK bonus and
keep 1 copy of your Rare+ for collection purpose.
It is good to note that your 2xHP Sidians are
worth 10x each! If you don't mind parting with them, you
could always sell them when the bonus ends.
DON'T sell your ★, ★★ and ★★★ Sidians for Time-Space
Powder as for the amount of Sidians needed you get only
1x. Instead, charge them at the Altar until you reach
300% and summon a S.Rare (★★★★★) card and THEN, sell
that S.Rare for 5x!
Here's why : a Rare+ (★★★★) card will cost you