It's not secret that the design of Dungeons & Dragons Fifth Edition makes spellcasters some of the most powerful characters a player can choose. While martial classes start out with strategic advantages over most spellcasters thank to a high Armor Class and a trusty weapon proving more reliable than a meager handful of spells, that changes at high levels. As a campaign goes on, spellcasters gain more options and spells that can shift the tides of battle in a single turn.
This can leave players who prefer martial classes like Fighter, Monk and Barbarian feeling left behind as casters grow and create a power gap between them. However, it's possible to design a martial character who is the perfect counter to Wizards, Warlocks and Sorcerers.
When designing an anti-magic character, it's important to play to martial classes' main advantages. Heavy armor and shields ensure as high an Armor Class as possible so that typical spell attacks cannot make contact, while stats like Constitution and Dexterity can increase efficiency in combat and protect against crucial saving throws that spellcasters can force. Since a shield will take up one hand, utilizing a weapon with finesse like a rapier can allow a melee fighter to use their Dexterity for their attack rolls rather than needing to invest in Strength, further spliting up their ability score allocation.
Dexterity offers another crucial advantage as well when it comes to initiative. This will be crucial for attacking spellcasters before they have a chance to cast while also amplifying the main strategy any anti-magic PC needs to focus on: closing distance. Bows and javelins simply won't be able to compete with high level spellcraft in ranged battle, but by establishing melee combat with a spellcaster a martial combatant can cut off their most valuable avenues of offense. Their ranged attacks suffer from disadvantage, while their area-of-effect spells carry the risk of hurting themselves as much as their opponent.
The addition of feats can lock such a melee contest into a surefire win for a martial class. Mage Slayer is an essential feat, allowing an attack as a reaction every time a foe casts a spell while in melee range, forcing disadvantage on their saves to maintain concentration and granting advantage on the fighter's own saves against spellcraft. In combination with Sentinel, which makes it incredibly hard for those in melee range to escape since any opportunity attack they suffer shrinks their speed to zero, getting a spellcaster into melee range can be a death sentence for them.
Such options are available to any class, meaning that there are a variety of anti-magic builds that will make enemy Wizards quake in their robes. If an anti-magic character is willing to utilize some magic themselves, spells like Silence, Counterspell and Dispel Magic are valuable tools that can turn their own tricks against them. The use of scrolls, potions and magic items can further leverage those advantages so that an anti-magic character can build an arsenal perfectly crafted to take on their enemies.
With the right, thoughtful build, an anti-magic character can be just what it takes to put a Wizard bragging about how much damage their Fireball did in their place. Everybody's pretty brave at 60 feet out with the rest of their party between them and the enemy, but put a thrashing rapier and shield in front of them with no method to escape, and suddenly that Fireball won't seem so powerful.