An entire group of 31s I was just with did the Hardmode up to the deathsinger. No 32. We are fine. Just 10 percent less damage than you 32s.
Edit: I get that 31s can't do the entire raid on their owns like a 32, I'm saying why decline one for not being 32. If they can't pull their own weight kick them. Some can some cant.
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We have had half our team 31s and had very little issues with Crota. As long as you work together and communicate your perfectly fine. Only recommendation is to have your sword bearer a 32.