Wizard 20 can single-handedly take over the Earth in XXI century. Fighter 20 is a man in a tin can with a big knife. In XXI century he can fight with a motorcycle gang, unless they got a granate launcher - and some do.
Think about the power level of a wizard and a non-magical fighter. If you think about what game rules are supposed to represent, a non-magical figher is a medieval knight, perhaps better trained, but on the same level. A wizard is equivalent of something from XXX century. Guess what, medieval knights don't exactly inspire terror in modern armies. On the other hand, an egg-head from XXX century isn't very good at swinging swords - but he doesn't need to be.
You can play that game, and you will always get Twain's "A Connecticut Yankee in King Arthur's Court ", with the knights blown up left and right. It is good for a bit of laugh at the idiots who agreed to play the knights, but nothing more.
If you want non-magical fighter to be able to fight wizards, you must pull wizards down to the level of actual medieval spell-casters, as described in romances, sagas etc.
That means no more Walls of Force, Gates or Wishes. On the other hand, you get to run around with a sack over your head to cause a blizzard.
And here is a short quotation from "Yankee"
http://bulfinch.englishatheist.org/yank/p9.htm#c43
"The sun rose presently and sent its unobstructed splendors over the land, and we saw a prodigious host moving slowly toward us, with the steady drift and aligned front of a wave of the sea. Nearer and nearer it came, and more and more sublimely imposing became its aspect; yes, all England was there, apparently. Soon we could see the innumerable banners fluttering, and then the sun struck the sea of armor and set it all aflash. Yes, it was a fine sight; I hadn't ever seen anything to beat it.
At last we could make out details. All the front ranks, no telling how many acres deep, were horsemen—plumed knights in armor. Suddenly we heard the blare of trumpets; the slow walk burst into a gallop, and then—well, it was wonderful to see! Down swept that vast horse-shoe wave—it approached the sand-belt—my breath stood still; nearer, nearer—the strip of green turf beyond the yellow belt grew narrow—narrower still—became a mere ribbon in front of the horses—then disappeared under their hoofs. Great Scott! Why, the whole front of that host shot into the sky with a thunder-crash, and became a whirling tempest of rags and fragments; and along the ground lay a thick wall of smoke that hid what was left of the multitude from our sight.
...
Now ensued one of the dullest quarter-hours I had ever endured. We waited in a silent solitude enclosed by our circles of wire, and by a circle of heavy smoke outside of these. We couldn't see over the wall of smoke, and we couldn't see through it. But at last it began to shred away lazily, and by the end of another quarter-hour the land was clear and our curiosity was enabled to satisfy itself. No living creature was in sight! We now perceived that additions had been made to our defenses. The dynamite had dug a ditch more than a hundred feet wide, all around us, and cast up an embankment some twenty-five feet high on both borders of it. As to destruction of life, it was amazing. Moreover, it was beyond estimate. Of course, we could not count the dead, because they did not exist as individuals, but merely as homogeneous protoplasm, with alloys of iron and buttons."
