One of the players in my game is the most fervent powergamer I've ever met. I know it's how he gets enjoyment from the game, and so far it doesn't intrude on the other players' enjoyment, so I've allowed it for the most part. Unless the loophole he finds is either explicitly disallowed, or would seriously break the game (either mechanically or thematically), I've let him use loopholes and combos to his heart's content.
However, whenever he makes a new character (especially a combat monster), it's difficult to plan around as the GM. I know that I could hard counter any of his characters by making an enemy or an encounter specifically designed around his current character, but that would be pretty noticeable and would take away from his enjoyment of the game without adding anything else.
So how can I provide a challenge to my powergamer or otherwise upset his expectations without taking away his enjoyment of the game or designing something specifically to counter him?