Mage killer

[zinycor]

How would you make a mage killer at level 11?

[Naanomi]

Monks are good at caster fighting... win initiative and stun-lock to prevent casting

Monster Slayer ranger’s best abilities (including foiling teleport) come online at 11

[Kvess]

It's hard to go wrong with a Way of Shadow Monk. There are many spells that can't target you if you are unseen via darkness or invisibility, Shadow Step is a free bonus action teleport, flurry of blows and stunning strike are great for breaking concentration and preventing spellcasting, stillness of mind and evasion can negate some of the nastier spell effects, and silence is just a cherry on top.

[Kane0]

Probably something dumb like Paladin 6/Warlock 5, picking up Mage slayer and then either fey or shadow touched depending on stats.

[ArmorClass]

Well there are two ways to go about this. With the feat Mage Killer, or without it.

For the Mage Killer feat you need to be within 5 feet of your opponent for maximum efficiency. So anything highly mobile and melee does well, a Monk with stunning strike like mentioned here will perform really well.
As an odd mention I would take 11 levels of College of Sword Bards. Counterspelling with jack of all trades is still rules legal to my knowledge. Misty step as a second magical secret makes you a really sticky threat, and makes it very hard for mages to keep you away from them.
You can also take 6 levels of Lore Bard for Misty Step and Counterspell, and 5 levels of Fighter. That still nets you extra attack, and also gives you more HP and action surge at the cost of spellcasting potential.

Without Mage Killer ranged weapons are the way to go. Bows are insanely good in 5e, especially with the Sharpshooter feat. The soft AC and HP of a mage often forces them to use terrain, crowd control and allies as defensive options. A sharpshooter ignores all that, and melts them down from 600 feet afar, probably in a single turn since -5/+10 from Sharpshooter is brutal against low AC targets.
The classes best suited for this are Fighter and Archer, as both gain fighting style archery. Fighter is better at it, but the new Ranger rules in Tasha can make them pretty much equally as strong.

The first built is a more flavorfull Mage Killer, the second built is a more effective one.
Choose what applies best for your table.

[mealar]

Kinda funny i literally came on here to look for builds like this,

rather than a build that is just good against casters i've been thinking of something thematic as a "spell-breaker". my standard base is a combo Wizard/Fighter, starting as fighter for armour and Con prof and then into abj wizard with the basic idea to lock down magic and then beat them round the head. the Abj wizard really comes online at lvl10 with the prof on counterspell so while a nice idea 1lvl of fighter may limit this build or may actually be enough. use the barrier and armour for defence and something like Booming Blade to attack with a warcaster feat to really lock someone down.

if you want more fighter the war wizard can work though that doesn't help with countermagic so much as let you augment you attacks when you do counter, but the extra AC/saving throw reaction doesn't matter much when you only want cantrips to attack anyway.

I can't give you an exact one as i came here looking for help for this too but hopefully my rough framework inspires some ideas for you.

[kingcheesepants]

The other posters have some solid ideas (a shadow monk or an archer). But I would contend that the best anti mage is in fact another mage. Namely an abjuration wizard. Their best anti magic ability (spell resistance) doesn't come online until 14 unfortunately but at level 10 they get to add their full proficiency to counterspell and dispel which is extremely handy for beating a mage. Add to that the fact that there are a number of spells that can really shut down a caster (in addition to the aforementioned counterspell and dispel, there is silence, greater invis, absorb elements, shield, see invis, banishment (to get rid of summons), private sanctum (needs to be cast before combat but very good way to ensure they can't teleport out), misty step (if they try to forcecage you), forcecage (if you can shut off their teleport). Take a race that gives magic resistance (yuan-ti, satyr) or half resistance (gnome). Then get alert and or a sentinel shield in order to win in initiative and you'll be well situated for taking down a mage in whatever situation you're in.

An honorary mention is the divination wizard because he can force the outcome of 2 die rolls (your initiative roll and your enemies save for example) and can thus win against just about anyone. I wouldn't put it as #1 though because of the very random nature of what you roll each day.

[zinycor]

Thanks a lot everybody!! You all certainly have me a lot to think about, but after talking to my party we considered that paladin would be the best fit.

Anyway, once again I thank you.

[Danielqueue1]

Also fun. Silence spell on anything good at grappling.

Way of the shadow monk with a rogue level or feat for expertise. Don't even bother stunning them, they cant use spells with verbal components (most of them) and most casters choose strength as a dump stat.

[JNAProductions]

Also fun. Silence spell on anything good at grappling.

Way of the shadow monk with a rogue level or feat for expertise. Don't even bother stunning them, they cant use spells with verbal components (most of them) and most casters choose strength as a dump stat.

Note that you can escape Grapples with Acrobatics as well as Athletics, and most casters will have a +2 or so in Dex. (Those that don't, like Heavy Armor Clerics, probably have a +2 in Str.)

Not that it's a HUGE deal, but it should be noted.

[MrStabby]

Level 11 just misses it... but if you thought you were likely to be playing a little higher then watchers paladin plus lore bard is nice.

Great saves, powerful ability to burn through hit points, Jack of all trades and counterspell. Given the good enough synergy anyway you can still just take paladin to 9, bard to two and just accept a nice boost on your first level up.

I would also throw in an honourable mention to the fiend pact warlock. Dark ones luck makes you a pretty effective counterspell. Throw in some bard again if you like, you only need 6 levels of warlock. Swords/valor bard 5, fiend warlock 6 would be able to suppress enemy images and best them in combat. Access to silence, invisibility, counterspell etc..

[stoutstien]

Rune knight has an impressive kit for taking out spell slingers.

[Angelalex242]

Let me give a shout out to Oath of Ancients Paladins in here. Damage resistance+saving throws adds up to sad panda spellcasters

[Greywander]

There are a couple ways to go about fighting mages, some of them mutually exclusive (especially at lower levels where you don't have enough class levels to get everything).

First is Counterspell and Dispel Magic, and there is none better than an Abjuration wizard. Bards are a close second, thanks to Jack of All Trades. There's also an odd multiclass combo where a bard/rogue can use Reliable Talent, making them even stronger than the Abjuration wizard (sort of, they have a higher floor but a lower ceiling). These two spells are all about removing spells that have already been cast and preventing the mage from casting any more spells. Without their magic, the mage is a sitting duck and will go down quickly. However, this is fairly expensive, as both spells are 3rd level. These spells are best used to support other anti-mage tactics.

Second is saving throws. There are some spells that use attack rolls, but the nastiest ones always use saving throws. Monks of 14th+ level and paladins of 6th+ level both get significant bonuses to all their saves. 2nd+ level War wizards also get Arcane Deflection, a reaction that gives +4 to a single save. Abjuration wizards also get advantage on saves vs. spells (but not other magical effects). As for races, yuan-ti and satyrs get Magic Resistance, giving them advantage on all saves vs. magic. Gnomes get advantage on all mental saves vs. magic, and vedalken get advantage on all mental saves period. Now, good saves will help defend you from enemy spells, but otherwise won't help your party (paladins get a special shoutout because they can share their save bonus via an aura), so strong saves are best combined with other tactics that force the mage to target you.

Next is melee optimization. Most mages are ranged casters, and will do almost anything to get away from melee. The Mage Slayer feat is an obvious choice, though not always the best choice. Generally, you're not only engaging the mage in melee, but also trying to prevent them from moving away, forcing them to target you before they can escape. There are a couple of different ways to accomplish this. One is with the Sentinel feat, allowing you to punish them with an OA and reduce their speed to 0 if they try to flee. Another is the Booming Blade cantrip, which still allows them to move away but punishes them with additional damage. Perhaps the most effective, however, is grappling. Grapple the mage and knock them prone, and they won't be able to move, they'll have disadvantage on all of their attacks, and you'll have advantage on all of yours as you grind their face into the dirt. There are many ways to build grapplers, but the most important things are to get at least one Extra Attack, get expertise in Athletics (rogue, bard, or Skill Expert feat), and get advantage on STR checks (barbarian Rage, Rune Knight Giant's Might, Enlarge/Reduce, or Enhance Ability). Also note that these aren't mutually exclusive: you can build a grappler that has Sentinel and Booming Blade. It should also be noted that many teleportation abilities will still work in these situations, so having Counterspell ready is a good idea.

The last thing we can consider is damage reduction. Generally, if you've picked up Counterspell, have good saves, and can stick to the mage in melee, damage shouldn't be as much of an issue for you. But it's still nice to have. Abjuration wizards get resistance to all damage from spells, and Ancients paladins get an aura that provides the same effect. Evasion will reduce the damage from AoEs. You can also pick up resistance to specific damage types. The tiefling starts with fire resistance, and can gain poison and cold resistance via a feat. Certain classes and subclasses offer damage resistance, perhaps the most notable being the Fiend warlock who gets a floating resistance that they can assign as they please. Storm sorcerers also get resistance, and eventually immunity, to lightning and thunder damage. A tiefling Storm sorcerer could be resistance to fire, cold, poison, lightning, and thunder damage, which accounts for a pretty big chunk of spells.

The last last thing is probably damage output. Once you've locked a mage down, there isn't much they can do, so you can take your time killing them if need be. But an alternative approach is to dump out so much damage that you vaporize them before they get a chance to do anything. There are any number of options here, including another mage with strong damage spells, but the one that comes to mind the most is the paladin who can make two attacks (or three with PAM or TWF) and smite on both of them. If your damage output is strong enough, you can almost ignore the rest of this post; a dead mage is no longer a problem. That said, if you fail to kill the mage in your initial assault, and you haven't fortified your saves and don't have anything to force the mage to stick to you, then you could be in for a bad time.

[MrStabby]

For reliable talent you will need a DM to rule that adding half proficiency is the same as adding proficiency, if I remember correctly.

[CheddarChampion]

Let me give a shout out to Oath of Ancients Paladins in here. Damage resistance+saving throws adds up to sad panda spellcasters

They also get spells/magic that can restrain, Misty Step to follow if they teleport away, and burst damage (Divine Smite) to make the most of it when in melee range.

Bonus action spells can be used to bait out Counterspell so your target doesn't cast Shield when you attack. But that's reliant on your opponent not seeing through this tactic.

[zinycor]

Once again I would like to thank you all, am constantly reading this as it is great inspiration.

For those interested I ended up going with a satyr (magic resistance and fey), from the azorius syndicate (calm emotions, ensnaring strike and counterspell) and vengeance paladin (paladin aura and vengeance spells).

[Eldariel]

Also fun. Silence spell on anything good at grappling.

Way of the shadow monk with a rogue level or feat for expertise. Don't even bother stunning them, they cant use spells with verbal components (most of them) and most casters choose strength as a dump stat.

It's worth remembering that there's a number of good spells without Verbals though. Off the top of my head Absorb Elements, Catapult, Counterspell, Hypnotic Pattern, Steel Wind Strike among others. The latter in particular is notable in that you can Teleport with it getting you out of the Silence and Grapple while doing stuff, and Hypno can of course CC the grappler. Just something to plan for.