Ah the good old stats vs hax matchup. SPOILERS FOR JOJO PART 7 BEWARE!
Armstrong is far stronger but he struggles to put Valentine down for good due to him being able to stall forever with his stand aka if/hen he gets punch, he drags another Valentine into the fight and repeat. Plus Armstrong isn't a stand user, he won't know what's even hiting him.
Valentine can also bypass Armstrong's durability by bringing a alternate dimension of Armstrong to him and it will defeat him no matter what. If someone brings up that Armstrong could use a Metal Gear and he can drive one, D4C's power also works on objects or in this case the Metal Gear so unless Armstrong is so much faster that Valentine can't react in time, he'll win.