There is no specific isos to be earned in bulk amounts. However the cosmic class catalyst quests n dual crystals may give u a handful if u so care. Better option would be too gather as many shards u can n pop open on cosmic dual shards day as chances of cosmic or tech ones are more in that specific day.
Depending the size of your 4* roster (chance of duping champions being higher the larger it is), dual class 4* crystals is the best way to get specific class ISO. Save up to 10-20K 4* shards and spend them on the crystal that contains the class you need. .
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Then let me ask. What’s the best place to get 4* shards for this little guy?