Use your spell that cuts down tapestries and ropes on the spiked ball. Walk up close to the edge and hold your trigger button (default R1), charge up for a half second and cast. Note: If you throw the imps into the fiery pits, they will die and you do not have to worry about them. Likewise, if you throw them all away you cannot put them into the cages to get out; but when all are indeed gone, they respawn from that hatch in the wall at the first ramp.