Just spent some time in monthly event quests. It will no give you too much but atleast enough and if you are good at farming shards then just go up for premium such as you can . Me too have completed act 4 only but from past 3 months i am continuaslly exploring heroic difficulty in monthly events and now am able to explore till 2nd chapter of master too without any potions. They will give so many shards , ISO8s,and catylysts. And a important point - do not open premium so hurriedly save them as much as you can and do a bulk opening like opening 20 in one chance. This will give you so many duplicate champions which is equals to a large number of iso.
I have 3×4* champions on r5 and yet i have more than 100 iso in stash
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I have 3×4* champions on r5 and yet i have more than 100 iso in stash