My impression from the books is that wizards aren't powerful in the 'chew up 20 orcs on a level field by themselves without breaking a sweat' sense. It's more that they have a ton of knowledge and wisdom. They expertly manipulate the flow of events across Middle-Earth, they can generally avoid getting into really dangerous situations, and have a lot of back-up plans and aces up their sleeves at all times.
When Gandalf finds himself in the middle of a goblin throne room, he doesn't just start slaying. He produces his notorious artifact weapon, the goblins go "oh shit!", he wipes out the lights, the goblins go "OH SHIT!", he lets it be seen when he strikes down their leader in the darkness, the goblins go "OH SHIT!", and then he slips everyone away in the ensuing shock and panic that he's expertly constructed on a moment's notice.
There are really only a handful of characters in all of Tolkien lore who respond to being overwhelmingly outnumbered by hacking away with abandon and slaying large numbers single-handedly, and they're always heroic last stands. In fact, Hurin, Feanor, and Boromir are the only ones who immediately come to mind. There are other characters who rack up large kill counts and win against overwhelming odds, but only when they're supported by armies on a battlefield. Remember when Gimli and Legolas have their kill count contest at Helm's Deep? They only get into the low-40's and that's considered impressive, but it's peanuts compared to the stuff the movies try to portray.