Yes, it was by design.
In the beginning, with lower ascention upgrades, you will need D20 dice to defeat the angel to farm feathers. Darkness and Light are in place to hinder the progression a little bit.
Eventually when you have decent ascention upgrades, your dice are powerfull enough to beat both the Angel, Darkness and Light, with lower level dice so you will farm feathers much more quickly.
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I like this game very much. As with the first version the only thing I want is more - more levels, content etc.
I'm not sure I understand your answer. It was by design but then with enough upgrades I can defeat them? I have all the upgrades and ascension upgrades both at 20 and can't do any damage to them.