I've had a few issues with the 3rd wave myself. The way I fix it is we wipe, then have two people leave and come back (if you're doing it with three) then the person who stayed behind passes host over to someone who left and joined. Then wipe again. That usually fixes it for me. For Crota, I'm not too sure, the only issue I've seen is when we leave a tiny bit of health left on him and we don't get the time to get the last hit in. But a lot of ways to fix your issue is by the host changing character. Leaving the game like that will automatically pass the host to the best connection. Often lag will be responsible for him not dying. Same as when you're smashing archon priest with Ghorn and his health seems to be gone, then it spikes back up to half.