Best way is to go back in time to the romance event and buy the iso selectors for 50 units.
Other options include buying iso from the glory store, doing the iso daily quest, doing the side quest at lower difficulties for 4* shards and hope for dupes.
One extremely good way to get ISO that goes under the radar because people are so focused on using them for 6*s is using the incursion currency to buy 4* crystals. You can get quite a few 4*s every incursion rotation doing this. It obviously depends on your roster and if that's worth it for you vs the better odds at certain 6*s, but this is an amazing way to get ISO.
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Other options include buying iso from the glory store, doing the iso daily quest, doing the side quest at lower difficulties for 4* shards and hope for dupes.
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