In a generic fantasy setting there exist "magic-users" who may, depending on their specialty (like any skill, you have to specialize to become competent), raise armies of zombies, cause literal firestorms, summon and command demons, turn shadows into physical substances, etc. They don't rule the job market because any fighting man may train hard enough to pull off physical and mental stunts that would be impossible in reality, allowing them to stand toe-to-toe with the magic-users.
How is itthis possible for training really hard to let you compete with magic-users?
EDIT: By "physical and mental stunts" I mean the sort of stunts seen in action movies like leaping across buildings, dodging bullets after being fired, ignoring the fatal convection next to a river of lava, resisting mental trickery, and so forth. Stuff that isn't particularly flashy but is impossible in reality like magic.