I found a quick way to Kill the Omni Stone Demon on one of the soul cord
levels.
When you are in the final room with the demon
wait until it is fully up then shoot a ranged spell at it a couple of
times. After about 3 spell casts (which hardly does damage but slows it
down) double jump then dash until you are right above it. Start hacking
away with your fast attack (if you use your heavy attack you'll fall
right through him and into the void were you will die). You might have
to dash back and fourth a couple of times but this will surley be a
quicker way to kill it.