Build - http://i.imgur.com/yH1MoHn.png + full cat armor (lvl 1) set and swords. Humans are probably the easiest yes, regardless I shouldn't be able to beat a dude 14 lvls above me without oils, pots and what not. Grey'd out the inventory count cause I am using the mod for it, incase its not allowed.
Yeah pretty much the build I had figured you'd be using. It is in my opinion the best (and also my current as well). The cat school skill and fast attack skills giving you 175% critical damage is pretty insane. Pair this with a decent set of Assassins Gauntlets (+10% Critical Chance) and your literally doing probably the highest damage in the game outside of a full charged Rend or using a full sign built Igni against a monster weak to it. Also whirlwind can proc multiple criticals I believe and seeing how many swings and arcs you get with it its easy to understand why you can put out so much damage. Using three red mutagens (how you ever go this many I'll never know!) plus full Feline gear (Im guessing this is what you have?) also gives a ton of damage.
But in any case this still shouldn't make the game trivial (and it indeed does in some cases). In the sake of fairness though your using in my opinion one of the strongest builds available. Everything is viable this is just superior to most in terms of damage potential. Also Fleet-Footed is to damn good as well. With proper timing you can literally dodge INTO large enemies and still take no damage.
They can't balance all builds and gear setups without obviously limiting choices. I'm not saying you should play a different way its just that you have conceived a very powerful build/setup which doesn't help when wanting a challenge out of the game.