I've been through it with 2x31s as well (I might have done it with 3, I can't remember though). The only requirement I have is that people have a tracking rocket launcher and heavy ammo synthesis and switch to their primary to continue shooting Crota after they shoot their 1 rocket (otherwise if there's a gap in damage his shield will regen), because if someone misses Crota and screws up the timing and him going to a knee it messes everything up. Level 30s are too low since they'll get one shotted from the boomers, but 31s are completely fine as long as they have a way to regen health, whether that be from armor or weapons. Fun fact: the first time I beat Crota on hard as the sword bearer, I switched gear to pickups orbs for health after the ogre phase so I went from a 32 to a 31, and then forgot to put them back on and didn't realize until after I killed Crota with the last sword lol.