Best way to use Magical Ruby CE exp card from event?

Most efficient way is to use them for CE bombing. https://www.reddit.com/r/grandorder/comments/8cpief/craft_essence_ce_bombs_for_dummies_rev_2_electric/ . They each give the perfect amount of exp if ur going by the guide and it's the most QP efficient way to level craft essences

Also, it’s probably best to wait to use them only when we have a Super/Great Success CE rate up.