(Re: the first couple of replies)
Sorry if I sound condescending, but isn't it obvious just by looking at the unit description that the Gallic Warrior is a horrible UU? The bonus means nothing. The Swordsman, which it replaces, is an offensive unit to begin with, so even if the Gallic ends up defending on a hill, thus making use of its bonus, chances are you didn't want the Gallic to defend.
Anyway, I have certainly noticed that the AI is using mixed stacks a lot more. I think this is the root of your problems. Before BtS, melee units were king because the AI seemed to only build Archers and Spearmen. But now, the AI spams Axemen itself, meaning it can counter Axes/Swords much better.
In the end, this means you just plain need more units. When the defender's stack is more mixed, it means there will always be a perfect counter to your attacker. The only way to overcome this is with numbers.