5e Mage hunter???

[Nosta]

What is a good build for a character who hunt mages?

Also range or Melee?

[Angelalex242]

Oath of Ancients Paladin.

Ask your DM about Mage Slayer.

[CheddarChampion]

Monster Hunter Ranger kinda flavors itself as a mage hunter. It works ok.

An Inquisitive rogue or an Arcane Trickster would work well. Lots of damage in one hit can result in a high DC to maintain concentration, add in Mage Slayer to impose disadvantage and you can get really good at "mundane counterspell." If you get an out-of-turn sneak attack with that same feat, you become really good at your job.

[Pandamonium]

Also, the Ranger subclass "Monster slayer" has some surprisingly good anti-magic user features at level 7 & 11. Level 15 as well but few campaigns go that long.

[SupahCabre]

Pick a monk. Particularly Open Hand or Shadow subclass, but any can work. Choose "Mage Slayer" feat as a Vhuman. Max Dexterity and Wisdom to 20 asap, then Constitution. Leave Charisma at 10 (or 12), dump the rest to 8.

Paladin isn't really a good mage killer, it's a good lich killer sure, but a high level Wizard would wreck a paladin

[Greywander]

Paladins get a bonus to their saves, though, which helps a lot against magic. Combine with a yuan-ti or satyr for advantage on saves vs. magic and you become pretty resilient. Oath of Ancients layers resistance to all spell damage on top of that. That said, paladins, despite their resilience, don't have some of the crucial tools for dealing with mages. Monster Slayer ranger also has some tools that help deal with mages.

As most mages want to stay away from melee, getting into melee with them is often a good first step toward shutting them down. As such, a high speed such as with the monk or a rogue using Cunning Action will be helpful. You also don't want the mage to escape, so I'd recommend looking into grappler builds. Grab the mage and shove them prone, then stomp on their face until they die. In this situation, the mage basically has no choice but to deal with you first, preventing them from targeting the rest of your party.

Two spells you'll want access to are Dispel Magic (to remove existing buffs) and Counterspell (to prevent them from casting anything else). Counterspell can especially be useful if they look like they're about to teleport away.

Classes to look at are Abjuration wizard (gets bonuses to Dispel Magic and Counterspell, as well as resistance to spell damage and advantage on saves vs. spells), Ancients paladin (adds CHA mod to saves, resistance to spell damage, smites can drop mages quickly), Monster Slayer ranger (bonus to saves, pseudo-Counterspell), and monk (rush into melee and Stunning Strike to stun lock and end concentration, Diamond Soul buffs your saves a lot). There are a lot of other classes and subclasses that can work, too.

[Zaile]

Ancients paladins DO have misty step though. And a Ancients/hexblade gets you EB and hex to their WIS or CON.

[Kane0]

Ancients paladin
Some form of Monk (probably shadow or open palm)
Walker Ranger
Abjuration Wizard
Some form of Bard (probably creation?)

[JackPhoenix]

See this thread. Reminder that Mage Slayer is a waste of feat.

[MrStabby]

I wonder if artificer could be worth adding to the list as well.

Firstly, it supports Gnome as a race nicely, with all of their bonuses to saves (i know many tables frown on yuan-ti)

Secondly, it has Flash of Genius for further benefits

Thirdly, it has a bundle of spells good for fighting mages - absorb elements, faerie fire etc..

Fourthly, lack of heavy armour is less of an issue if you are wanting to be strong vs saves rather than attack rolls.

Fifthly, you can adjust your infusions to counter the specific mage you are fighting

You can build it up with things like mage slayer (only worth having if you go melee), with feats like fey touched (misty step is a bit of a must fighting mages), or just focus on surprise (infiltrator armourer) and taking them down with damage feats like crossbow expert/sharpshooter before they can react.

[Eldariel]

Yeah, Jack's thread is pretty good. Long story short, Lore Bard and Abjurer Wizard are probably the best Counterspellers in the game and since Counterspell is absolutely huge against casters, there's that. OTOH classes with Initiative bonus such as Diviner and Chronurgist work really well too. If you want to hunt casters, your best bet is generally going to be being a caster yourself since this edition doesn't really give good anti-caster tools to non-casters. Best they get is buff to saves...but on a level where casters don't really care about your saves that much anymore.

The one approach for non-casters is going superlong range and trying to engage casters in arenas where you can utilise being able to fight at over 150' range (basically Sharpshooter or Spell Sniper Eldritch Blast).