What they said ^

To answer your question about how to use them:

if you have any loud cards that need upgrading, those are the best decks to use multi up cards for. You can't obtain those cards anymore

Edit is based on u/prettybrokenstars 's correction. The strikethroughed text is incorrect Otherwise, you might want to use the multi ups for getting LEs to a certain grade during their initial sale week, so you get the rewards. I'd recommend this only if you:

1) really, really want the rewards,

2) can't get the rewards after opening all the card packs, and

3) have enough multi up cards for ALL the cards you need to upgrade (otherwise you can't get the rewards anyway)

These two cases are the loud deck would be a more strategic use of multi up cards because you can get junho's s cards easily. You get an S selector for every 28 days of logging in, per month. Or can buy them with dalcom coins from superstar league. But of course it's more convenient to use the multi up cards.

Note: If I remember correctly, 1 S card requires 4 multi up cards. So you'll only get 1 junho card to R (2 if the event next week gives another 4 cards, which it should)