Playing vs hard ai britons

I play celts, Every time i play against hard i defend their first attacks and got sweeped by the massed briton archers in every game after we reach imperial age even if I feel like I have the game and push almost to end they slowly start to mass and I am terrible at micro so they find a way and snowball out of control, how can one overcome them? I am a new player, I beat moderate AI everytime but against hard ai I am hopeless, any suggestion would be appreciated.