Here's the original JP quote:
赤EXはある程度戦力の整ってる人なら ばソロ攻略も可能ですが、青EXは高レ ア回復神姫や英霊で固めた廃課金パ ティでも容易に全滅し得る程の異常 難易度となっており、ソロでの攻略 は完全に運次第となります。
And the translation:
While the Red Expert boss can be soloed by a decently-equipped player, the Blue Expert boss is abnormally difficult, and even a whale with high-rarity healer kamihime and souls could end up annihilated. Successfully soloing the fight comes down completely to luck.
As for the difference in rewards, there's two types: those earned by reaching token milestones, and those obtainable in the event Gatcha. The Blue raid drops tokens that lead to a copy of the SSR weapon and Gatcha tickets that include the SSR eidolon as a potential reward. The Red raid drops tokens for the SSR eidolon and tickets that give you a chance for the SSR weapon.
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As usual, all enemies are the same element as the titular eidolon.