The only hunter of the giant squid that we know of is the sperm whale. Sperm whales dive down to the same depths as the squid, and scientists used their knowledge of the sperm whlae to lead them to giant squid. When whales attack a squid, the squid will suction its arms and tentacles to the whales head, often leaving scars. Many fisherman have witnessed this and have also observed the marks on their catch.