For many years, nobody knew what the giant squid utilized for food. This is because they have never really been observed in the wild. Some recent studies on dead individuals have shown that giant squid eat deep-sea fishes, such as orange ruffie, and hokie. They also eat other types of deep-sea squids, but not Architeuthis, the giant squid.
As large as these animals are, they would probably be able to capture almost anything, maybe even whales(see comments)! They capture their prey by using their two long feeding tentacles. The tentacles are shot out to grip the prey. The suckers on the tips of tentacles grab hold of the prey and the tentacles contract, bringing the prey to the arms. The arms then further subdue the prey, pulling it to the strong, sharp beaks. The beaks are operated by a massive set of muscles that allow them to bite through just about anything the squid might capture. But the giant squid's bite-sized pieces of food need further shredding before being digested. The tongue is equipped with an organ known as the radula, which is loaded with rows of small, file-like teeth. The radula further shreds the meal before the tongue pushes it down the esophagus to the digestive organs.
There are very few predators of the adult giant squid. The best and probably only one is the sperm whale. As babies and juveniles, they have many pedators, mostly deep sea fishes. Once giant squid get beyond a certain size, they have outgrown the size of most of their potential predators. Sperm whales grow to 40-50 feet in length, but they weigh 30-40 tons. So even though a giant squid is huge, it is not big enough to escape or to fight with a sperm whale. Most of the time the sperm whale wins. This is evident in the number of giant squid found in the stomach of the sperm whale. (Forch 1998) (Banister and Campbell 1985)