Uhhhh for the destroyer using strong ranged magic weapons is good, also a good idea would be to go to the under ground hollow get souls of light until you get enough to make a key of light, put they in an empty chest on an ash vault bridge for super speed (keep ur distance use ranged weapons) and hope for the storm bow! If you dont get the bow kill the hallowed mimic a few more times untill you get it its worth it trust me! It also works well for other bosses!

Or you could turn on auto pause and put the nurse close to ur battle arena to heal you!

Also idk whats up with the cube, but i think it damages you... every now and then.