I didn't touch hard Crota until I got my Warlock to 32. I also didn't do it on my others until they too were 32. Instead of being told "no" by a 32, just get to 32 by doing normal mode. I guess it just seems to make sense for me to take on the "hardest" part of end-game when I am at least at the recommended level. Unless you intentionally trying to go for a level 31 only run (for Crota).