The best way of tenderizing meat is still to attack it physically. The most extreme method, grinding, will shred all the muscle tissue into tiny bits. This is what makes tough cuts of beef into tender minced beef. Pounding will also break down muscle tissue, but it is really only good for thin cuts.
The chemical method, applying marinades, is less work but less efficient. Commercial meat tenderizers or marinades may contain an enzyme such as “papain”, which the ancient Mexicans used without knowing it by wrapping meat in papaya leaves. This enzyme breaks down muscle in meat. Acidic marinades such as those made with wine or vinegar can also do this. This catch with both, though, is (a) that they will really only affect the surface of the meat, and (b) because they dissolve structural tissue in the meat which held water, the meat can be drier. You can help to get the marinade inside the meat by poking the meat with a fork first, but this may only help the meat to lose more of its juices when cooked. Tenderizers can also make the surface of the meat mushy while leaving the inside tough, and may give a flavour you don’t like. For marinades, you can buy kitchen needles that inject it into the meat. You can also investigate getting a home meat tumbler.
But overall, for tough meat, the best solution is just to cook it properly, with long, moist cooking such as stewing or braising. That is the reason pot roasts and stewing hens spend hours in ovens and crockpots.