For all who is tryin to lvl up sub weapons go to survival mode lvl 1 hard equip your sub weapon to down b then equip astral ring with a master ring, start level hold down and alternate from b and lb then die when u have no more hp u rinse repeat...
ps: i cant take credit for this some1 else told me
That doesn't work with all sub weapons. Take the Bible sub weapon for example. If you spam it all at once, it only hits the boss just like you threw one. It only hits the enemy every second. Doesn't matter how many of the subweapon is out there. The important thing with the Bible is a continuous flow of books so you don't waste time (time it takes to die and respawn).
Also, the axe, it's better to just throw one at a time. If you place it right you can hit three times with each axe. If you spam the axes really fast, they generally only hit the boss once per axe.