How to tenderize large squid?

I agree that a long braise would be one way to go but I also use large squid to make one of my favorite chowders, faux conch chowder. I use my meat grinder attachment for my KA stand mixer to grind the squid, tentacles and bodies, and then combine them with chopped clams in a traditional Caribbean conch chowder broth. I find that the ground squid mimics the texture and nice chewiness you would get with ground conch. My family and friends love it and if I didn't tell them, they would have no idea that it wasn't real conch in the chowder.

marinating over-nite helps to keep it tender. Ive also seen people dipping squids in hot water for couple seconds then taking it out and repeating it for few times to keep it tender before actually cooking it.

Kiwi fruit. Works so well don't do it too long!

Papaya is a known tenderizer for octopus, squid, and other meats. Another option is papaya enzyme tablets found in the supplement section of drugstore. Either can be added to the recipe while marinating or cooking. The tablets have the advantage of imparting no flavor. I can't judge the amount without a recipe, but pretty hard to go wrong. Also squid and octopus must be flash grilled or cooked for a long time. Anything in between results in tough meat.

You might try using some Korean pomgranite wine as a 6 hr or more marinade or try a dry rub with baking soda and wash it off in 2-4 hrs.

In Africa they tenderize them by beating them with rocks, like you would do with a steak tenderizer. A bit of manual labour, but well worth the effort. The by far most tender calamare I've ever had was in Namibia, and that was beaten before cooking.

"I use buttermilk (enzymes in the milk) to tenderize: squid, boneless skinless chicken breast, and boneless pork loin chops. I soak for a couple hours or so and it gets rid of all the "fishy" flavor. The lactic acid in buttermilk dissolves proteins in the same way monosodium glutimate (MSG) does. It makes the meat break down without the rotting process. I soak my steaks and roasts for 8-12 hours and my gamey tasting meats like some fish and venison for up to 24 hours. Just rinse off and cook.