Just finish all the reward stages, so I’ll try my best to answer my own question.
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Worth to do it? Yes.
so many Shards and Treasures, with extra Vault Key, Diamonds, and 2 Minor Chaos Orb. -
Minimum Tier? Just 2 (90 gems)
Except you need a new weapon, then go for it. -
Best Strategy? Clear all stage until level 100, then stop.
Use your best team for it. In actual Underworld, it’s easy enough to farming for end-gamers and doing fraction challenge.