The most difficult aspect of the Diamond Cooksta rank is the taste rating of 375 which can be done the easiest by upgrading the Narwhal Sushi to lvl 9.
Go down to the glacier twice a day, take a tranquilizing gun like the Tranquilizer Rifle and harvest the Narwhal with the submarine. There is always one Narwhal in the upper layer of the glacial area. One 3-Star Narwhal gives you 27 meat.