I'm not judging or hating on level 31's because not everyone plays all day or you could be unlucky to not have boots for your titan like myself but I will say after running crota on hard last night.....31 probably won't cut it in most cases. A 32 with maxed black hammer or ice breaker is taking 5 to 6 shots to kill yellow bar knights. You'll get one hit by boomers, and just overall take more damage. It's tricky to regen health against crota so taking little damage is crucial. Now by all means if you're highly skilled and know how to stay alive when you're not killing things quick then maybe you'll be fine. I can tell you that I won't take my titan in hard mode until he is 32 like my Hunter and warlock