There are 27 caskets on each side of the room (54 in total), since Ico broke out of his you must kill 53 shadow spirits - previous horned boys who have been transformed by the Queen to do her dark bidding.
Instead of running around all over the place trying to catch them, just stand in a corner and they will come to you like lambs to the slaughter!
You make about 1 kill every six seconds this way... the last one got stuck on the stairs so I stood there waiting for it to show which took longer.
There's no modding in this video - I was playing the PAL version here.