Play the Vortex Gates on different days to earn them and other bonuses.
Monday’s daily gates will earn a ton of Karma.
Tuesday’s will earn you Nymphs, Spirits, and Icons, which are used for basic evolutions.
Wednesday’s vortex gate will earn Mimics, which are used for evolving slightly more rare units.
Thursday’s battles will get you Totems and which and they can be used for evolving four-star super rare cards.
Friday’s battles earn you a ton of EXP and craft materials, while weekend battles will earn loads of Zel.