This isn't even a hard fight. I'm legit confused at what's difficult. Look at where the first sin eater goes. Stand on opposite portal. If it goes through red first, stand on blue and vise versa. If you're doing colored part, then all you need to do is see where umbral/astral is going to go by seeing where your designated sin eater is going. Wherever it goes, you stand on opposite. It's very straightforward.